I currently am playing the game via Xbox Game Pass Ultimate on PC. Do I need to purchase the full game as well to play the expansion or can I just buy the expansion and I will be ok for VoH?
Yes - Microsoft Xbox has always required full game purchase any time they make an expansion available. I’ve seen this for other games. A good example of this was Outer Worlds (which is why I never purchased the expansion).
The good news is, the game is half the price as release and you can get a combo back for both game and xpansion. I had to pay $69 when the game released so… trust me, it could be worse.