In the Curse of Strahd campaign, does anyone know approximatley where the country of Barovia was located (with relation to other well known locations in D&D) before being transported to the Shadowfell? 

Barovia was never in the Sword Coast, Faerun, Forgotten Realms, etc. Ravenloft was the setting/location where Barovia was placed.

This link should help: https://forgottenrealms.fandom.com/wiki/Demiplane_of_Dread#Barovia

Ravenloft is the name of the castle, is it not? 

My confusion is on where was Barovia BEFORE Strahd came. According to the CoS manual, Strahd's army conquered a valley civilization. He named the country Barovia after his father, King Barov and his new castle after his mother, Queen Ravenoia (I apologize if I messed up those names, I do not have the module in front of me). All of this occured even before Strahd became a vampire. 

So where was this valley located before the mists engulfed the country of Barovia? 

The short answer is; it is not clear.

Quote from Kaboom979 >>

My confusion is on where was Barovia BEFORE Strahd came. According to the CoS manual, Strahd's army conquered a valley civilization. He named the country Barovia after his father, King Barov and his new castle after his mother, Queen Ravenoia (I apologize if I messed up those names, I do not have the module in front of me). All of this occured even before Strahd became a vampire. 

So where was this valley located before the mists engulfed the country of Barovia? 

Barovia did not exist outside of, or before, the Demiplane of Dread. It was due to Strahd's treachery that the area became his domain and became Ravenloft proper. 

If you look at the lore in a slightly different light: Barovia existed, before it was claimed by Strahd's dark powers, in the Demiplane of Dread, a vast plane of existence that is, in itself surrounded in the magical mists. (lore, spoilers, etc.)

Barovia was originally part of a forgotten world. It wasn't part of any of the popular D&D worlds mentioned in the core books, and as such you won't find much lore for it beyond the few bits and pieces spread all over Curse of Strahd, such as references to the Holy Empire of Valentia.

When the original Ravenloft came out it was a stand alone module for AD&D. this was just before Forgotten Realms came out. I have the original module that all this started from. According to it players are magically teleported to the lands after being lost in a forest. I placed the town of Barovia and Ravenloft Castle outside the Valley of the Mage on the Map of Greyhawk. Now I am playing 5e and getting ready to run Curse of Strahd. I am also looking for a place on Toril to place it as I agree that it was once a "REAL" place on Toril.

Most of the time these kinds of things are left intentionally vague so DM's can put places wherever they see fit in any world. Barovia could easily be a vast, Orzhov-controlled neighborhood in Ravnica, or just as easily be a small mountain nation in Greyhawk.

I never played in, or ran, the original Ravenloft modules. However, mnay of my current players were involved in them and told many stories over the years.

I just took what they have told, talked to a DM who had ran the modules, and ran with it. The DM who I talked to had placed Barovia in the World of Greyhawk. I ran Curse of Ravenloft in the Plane of Dread. A couple of the characters were children of adventurers who were in the original module. One was a wizard who escaped into the Mists to escape enemies and was lost until now. He had fought Strahd before. However, the Mists had drained him of power (levels). He had his knowledge and some gear, but had to rebuild his strength anew. Others of the players knew only tales handed done from a parent.

Whenever I run a module, I always try to fit it into the context of my ongoing campaign universe which I started back in 1981.

Not sure where, seems like near Neverwinter if you start cross referencing information. However, going back to what someone said above, this campaign was written before faerun.

The Ravenloft module was written by Tracy and Laura Hickman (later known for Dragonlance), and like most early adventures, was not explicitly set in any campaign world; while the World of Greyhawk (basically Gygax's personal campaign setting) had been printed by the time it was published in 1983 (though apparently Ravenloft was first written in 1978), plenty of games didn't associate with it, and the Hickmans, being based out of Utah, had no particular connection to that setting. Thus, it, like the Desert of Desolation series, is set in 'generic D&D location'.

At least two of the adventure hooks in the 5e module start the campaign near Daggerford. So there is at least likely an entrance to the Demiplane of Dread there. Not sure if that means that Barovia was once in that area, but it's possible. It seems strange that an entire valley (complete with mountains) could just vanish, but there might be some kind of magic at work. 

Like, maybe it's like a cloth that you pinch with your fingers. Turn the cloth one way, and you have a semi-flat shape; you can easily rub a finger from one edge to the other despite the fact that the fabric technically had a larger area between the two points. That side would be Faerun. The other side, with the pinched material, would be the Valley of Barovia in its Domain of Dread on the Demiplane. 

Or maybe it would be more like when you mend a garment that has a hole in it. The hole is where Barovia was. However, just the nature of the world wouldn't allow an emptiness to reside in the fabric of reality, so the natural order just "sews" that hole back together without putting anything in its place. Thus, the material around the emptiness is pulled together, a sort of stretching to cover the hole. But people's perception can't register this, and/or reality smoothed out the edges. So there is no sign of it aside from the paths into the Domain (gaps in the stitching). 

I recently plopped it on the Western end of the Southwood between the High Moor and High Forest, up the river from Daggerford.  It brought its terrain with it, obviously, and so the Greypeak Mountains might now extend out and into the forest.  Also the Moor would mean the valley will still get a fair amount of mist naturally ;)

Would it work to put Barovia in the region of Exandria now known as the Dwendalian Empire? In the Wildemount Gazetteer the author says that the Dwendalian Empire was based on medieval Russia and Barovia is very much a Slavic culture (though probably more Romanian than Russian).

Definitely a valid entry vector for adventurers to go into Barovia from Exandria.  In terms of where it would re-appear in reality if you somehow broke the curse on the land....  At this point I guess it could theoretically be anywhere since apparently its original world theoretically no longer exists (???) but you'd probably want to establish a post-Strahd narrative through a campaign run with an Exandria party before doing so.

I feel like the defining part is the mists. It can be wherever you place it. I had the mists come to the characters caravan whilst traveling through the Storm Lords on the way to Sembia. 
The mists won’t let you enter the area, in any case, without approval from Strahd or unless accompanied by Vistani. 

The module does mention, in one of the hooks, the party being near Daggerford. So if you want a place, that’s as good as any.