I'd say "That wish is going to screw your character over hard if you make it. There is no scenario where you just get vampire powers and carry on like normal. Do you want to proceed?"Would no one just have the spell not work or just say no if they're so concerned about the vampire PC?
I have enough old-school in me that I want the "wish gone bad" to be a thing in the game world. But I don't want to "gotcha" the player. If the player wants to roleplay the consequences of a wish gone bad, I will try to come up with a fiendish but fun interpretation that provides some cool story hooks.
If they just want a huge free power boost, I'll let them know that's not happening, and they should scale back the ask to, say, a magic item of comparable power to a 9th-level spell scroll.