I feel like this might be a very stupid question, but I'm gonna ask it anyway.

In the next book, Van Richten's Guide to Ravenloft, players are supposedly going to be able to OBTAIN the Dhampir, Hexblood, and/or Reborn "races"/lineages. How would that affect a player character?

Example, I have my players coming up with character ideas. I have one friend thinking of a Reborn Tiefling Bard, and my younger brother is thinking of maybe an Dhampir Aasimar Paladin. Does the VRGtR lineages COMPLETELY overtake the original races? Or are there some things that would still carry over?

Yes, the lineage overtakes the original race. They can for example be a reborn bard, or a teifling bard. But not a reborn tiefling. 

As presented in the UA yes, when you take on a lineage you basically swap it for your race.  A _lot_ of players don't like this, but also a _lot_ of players see it as a balance need.  So this thread will be mostly conjecture from the UA and we'll see how much of the published book still reflects them.  In the character builder here in DDB, if you take a Gothline, you're swapping races.

Good point midnightplat. My answer was based on a quick read of the UA, but the final book could be different. 

So basically they can be a reborn bard that LOOKS like a Tiefling, but without any of the tiefling traits. Ok.

Thanks everyone!