So I look this up, the game is split up into chapters. The first campaign is 10 chapters, and each one is at +10 sessions. What does that mean? Is each chapter it's own story? The first chapter is about saving a npc from the underdark while the next chapter is about going to another country. I want to do something in my campaign where it's the players fighting and running different stories. One chapter is about fighting a cult the next is about foiling a a end of the world plot. So how did Matt Mercerer do it? Was it that each chapter was connected to the eventual resurrection of Vecna? Is each BBEG just Vecna's grunt? Does each chapter just foreshadow his return?

essentially, each chapter is a separate story arc contained within each campaign. not everything is actually connected, for example the first arc in campaign 2 has absolutely nothing to do with whats currently going on, other than that's when all the characters came together.

If you think of it like a tv show each season could be it's own "chapter", with the series run being "the campaign." Maybe the 2nd chapter focused on the quirky new neighbor ho eventually becomes the friend that saves their life 3 seasons down the line.

So my next question is how can he keep each story arc so short? The stories span to 5-18 sessions at most. The final Vecna arc only span 15 sessions. 

It helps that he doesn't follow the guidelines for number of combats per day. There's roughly one encounter between any two interesting locations and one to three encounters for a given location.

So theres less combat so more progress. Is it wrong to have a story that runs longer then 15 sessions?

A couple things to note about Critical Role:

There is always a looming timeline established. The arc almost always has at least the appearance of a semi-living world and not a static, reactive world. This means that things progress along a vague timeline regardless of what the players do. This tends to keep the players focused on the immediate goal instead of wandering around "side-questing". If they take "too long", then they could very well just automatically lose, or at least make succeeding exponentially more difficult; or if not more difficult, then with serious consequences to the world (The Briarwoods can eventually consolidate power, the dragons can eventually establish a solid foothold and expand their control, a god can eventually achieve his goals).

With that in mind, the players don't mess around a lot. They have their fun, but tend to keep to this vague timeline so they should also get a lot of the credit for "keeping things short". This was especially apparent in the final arc where it was entirely possible for VM to see ALL the gods and gather ALL the vestiges, but by then Vecna would have won.

As for the "ideal story length", the story can run as long as it runs... there really is no limit of whats "good" or "bad". It only becomes a problem if you notice that your players are starting to not be as invested in the current story. 

Also, keep in mind, the "chapters" of Critical Role - the only place the story is divided into "chapters" is on the fandom wiki. So the chapter divisions are decided on after the fact by fans editing that site, and not really set out by Matt Mercer. Sure, the final Vecna "chapter" is 15 episodes. But they've been foreshadowing the Vecna stuff since Chapter One with the horn of Orcus AND the sphere in the temple underneath Whitestone during the Briarwoods arc. Likewise, the Briarwood "chapter" ends at episode 38 on the wiki but ***SPOILERS*** they keep showing up all the way through freakin' episode 114.

The divisions aren't nearly as clear-cut as the wiki would have you believe. Just like in a book - character's don't necessarily show up in just a single "chapter".

Stepping out on a limb here, waiting for someone to chop it off ...

I think using the terms Chapter, Arc, Foreshadowing, etc. - using the language of the structure of Novels - is problematic here. It implies that there's an overall narrative structure that Matt Mercer writes out ahead of time. I doubt this is the case - especially given his reaction in the current Mighty Nien campaign where they pulled out the Luxon Beacon and surrendered it to the Bright Queen. I'd have to go re-watch the end of that episode, but if I recall correctly it was along the lines of "OK, I have to go re-write a lot of notes". ( which I interpret as meaning that the world will need to adapt to the Party actions, not that there's an overall pre-determined story structure that the Players are now "messing with" ).

I think Warranto and Maestrino have pointed out aspects closer to the truth: a) Matt's world is dynamic, and evolving on its own schedule and terms, and b) any structure that arises is post facto imposed by the chroniclers of the show.

If you're running the world that way - essentially building the entire world's complex background, and then role playing it in real/game time - there is no plan for chapters, and no plan for how longs chapters run - since there are no chapters, and no concrete story plan. There is still definite direction and flow, however; it's not an amorphous sandbox. The various NPCs and Factions all have goals and agendas, and they act according to their goals, character, and knowledge. The plot won't ever founder, because even if the Party sits around doing nothing, the world will go on without them - and the Party may lose.

It's a development method that makes things look way cooler than there were planned to be. You asked "Was it that each chapter was connected to the eventual resurrection of Vecna? Is each BBEG just Vecna's grunt? Does each chapter just foreshadow his return?" - and the answer is both yes, and no. Was it planned that way, from the beginning? I don't know Matt, and can't ask him - but I doubt it. I suspect that what happened/happens is that Matt has an overall general direction he wants the story to go, and then weaves the narrative that the Party creates into it. That means that it ends up looking like BBEG #1, and the events of session #22 were all created as foreshadowing for the climax of the campaign, but what really happened was the climax of the campaign arises from the interaction between the actions the Party and the actions that Matt decides the various BBEGs, NPCs, and world Factions are going to take in real/game time. It might look - in retrospect - that Matt built an ending, and worked backward to create amazing plot tie ins way early in the campaign, when what actually happened was events that happened early on in the campaign (which no one planned at all), get woven into the conclusion.

You seem to be wanting to build a story structure similar to Critical Role - but I don't think there is a story structure in Critical Role. Instead, Matt Mercer is very good at taking the unplanned events that arise out of play, and weaving them - reactively - into the overall story line, to keep the overall adventure from fragmenting, and giving the Players a real sense that their Characters, and their actions, are important to the overall flow of the narrative & to the fantasy world around them.

I've only started watching Critical Role during its second campaign - I'm far too lazy to watch the first one. 

As far as I can tell, Vedexent explained exactly what I have personally perceived - the campaign follows the characters actions and decisions.

I think it's important to remember that everyone at that table is a talented actor, writer, and creator. They all know how stories work and flow. Ultimately, I think it's an unconscious result of their experience and talents that make the story move in the way it does.

The only advice that I can give you is to not try to make your campaign like Critical Role. Again, they are professionals, but more importantly they are a unique group of individuals that come together to create a story (not to mention have too much fun). You should just focus on running your campaign in a way that makes sense for you and your players. Don't rush them, but also don't let them flounder. Keep them engaged, but don't push them along.

@Akacen

I think you've hit a good point about the Player skill and the overall group dynamic being the true strength of the campaign. Stop and think about how much of each session is merely the Characters talking to one another - something on the close order of 20%, I'd estimate. Also look how complex those Characters are, and how rich their motivations and goals are, in some cases. 

In some ways, I think Matt Mercer is spoiled as a DM, having a table of Players that good at storytelling, and so dedicated to it :) 

I think many people would like "the Critical Role experience" when it comes to playing D&D - but don't recognize that the unique game that is that show is a blend not only of Matt Mercer's storytelling and DM talents, but also the Players' acting skills, and the larger social dynamic of that group of friends.

So - if you want to replicate the Critical Role experience, all you need to do is become a world-class world builder, develop some serious acting ability, and form a gaming group of your close friends ( those that you've known for 10 years or more - the kind you would leave custody of your children to if you were killed in a car accident ), and who are all equally dedicated to creating and playing rich and nuanced characters, and building rich and complex story lines.

Sounds easy when you put it that way ;)