How about this: the common tongue as mentioned is.. more or less, the common tongue (haha.)

Then you have upper class speech, which is probably Dawnish. Even Vasili noblemen speak this language because, well, it's prestigious and it reflects well on your status. See it as the Latin or French of Europe. Why Dawnish? Because they seem more noble-like, where as the Vasili mingle with nordic tribes.

Besides that, you have tribal speech, which is subdivided into.. over a hundred different dialects as each tribe speaks it's own dialect, and has certain words that the other tribes only know if they've met this tribe in a friendly manner. 

Further south you have many more languages, mainly used in trading, where they speak a mangled version of the common tongue, mixed with some of their native language. Ofcourse, this goes paired with lots of hand gestures which, over the years have also formed 'words' so you can use your hands to say words. For instance, if I wave at you and then pinch my nose, that means 'I want to buy sugar from you' for example. That's a dumb example, but it gets my point across.

Would that make sense? Just an idea that I came up with because, in England they spoke French as well because it was seen as a better, more noble language. It makes sense for us to have a similar situation.

I also wanna ask what the currency is in Vasili and what it's called?