Has anyone run Dragon Heist and kept flipping on the villain?
I'm in Chapter 4 now, and they have just gotten started.
With the regular content, the party has managed to get a brewing business and rat races running in their tavern. 
However, the sorcerer was brewing demon blood, and his character got arrested. For burning down the factory where you find the Bandit leader, with the two Kenku.
I was really thinking on going with Manshoon for the villain, but with all the Devil-related stuff going on anyway, and the rivalling criminal networks fighting anyway. It definitely feels like the Cassalanters would be good villains at this point. Any idea on how to bring them in, I feel the reveal at the end will be deflating since they really didn't get a lot of detail on the villain going through?

Chapter 4 as written is just awful anyway. A 10 scene "Chase the Macguffin" that ends when your players can't take it anymore is not anything close to what I call good writing.

I had multiple villains active (Xanathar and Jarlaxle) when I ran the adventure, since it makes sense that they would each be interested in the vault.

I ended up pretty much whole-cloth homebrewing everything until they got to the actual vault. It was a while ago, but here is what I did:

• A side quest given by one of the player's backstory character where they had to stop an adult oblex from taking over a shop (forshadowing the planned transition to Out of the Abyss after Dragon Heist)
• Xanathar's Kenku minions attack the Trollskull to try to get even.
• A NPC from the Harpers (one of the factions a player was in) tells the players of a fancy dress party that Jarlaxe may be at (I forget why this was here, I think it was how I got them the information about the keys needed to open the vault, or it could have been to get one of the keys)
• 3 adventures, one for each Key
  • A mad person's sanity (either Xanathar's goldfish or Jarlaxel's lever's paintings)
    • option 1, which they chose, was infiltrate xanathar's lair and take the goldfish
    • option 2 is the converted windmill (Autumn)
  • A gift from a queen (Larael Silverhand is the Witch-Queen of the North, the party used the library in Trollskull alley to discover this)
  • The factor of the common folk (I made up some carnival games for the slums on the north side of town and they could win an [magicitem]alchemy jug[/item] for completing enough events)

After they got each key I then proceeded to the vault.

So my suggestion for how to run it is:

• Something happens related to the Cassalanters, maybe they buy up a bunch of the shops around the players (or the player's own shop) under dubious legality and the characters have the steal the deeds back.
• A side quest or two related to the player's interests and backstory
• The players can investigate Mansoon and reveal some detail that forces the city to remove him from Kolat Towers (or the players can if you think they are strong enough / they have help)
• Short adventures for each key they need
• Vault of Dragons

Thank you very much, that's exceedingly helpful!