Well the problem with any "redemption" story arcs is that that is meta-gaming. If you don't engage in meta-gaming, then you should be role-playing your character in the here and now, and reacting to the things your companions say and do in the here and now. And in that role-playing I don't see how a good-aligned PC can justify continuing to adventure with a vampire or a Sharan beyond the necessity of the first bit of the game. It's not like Volo comes to you tells you: Hey, don't be concerned. These companions can become nicer/good later on the game if you keep at it with them.
Yes you are right but in my head my wizard is thinking that she sees the good in shadowheart and is secretly hoping to bring her into the light. Likewise my cleric believes that if Astarian can get free of his master then he can be resurrected and redeemed. (which, from memory, was one of the possible outcomes of the original Strahd module). I don't have high hopes on the second.
