Can someone cast Locate Creature on a creature whom they knew when it was alive, but died recently and became undead? For instance, if a party member was bit by a vampire and died, could someone cast Locate Creature to find the person?
No, but ask your DM.
This isn’t a hard no. This is a “I would rule no, but your DM may rule otherwise.”
The reason I say no is in the spell description of locate creature:
If the creature you described or named is in a different form, such as being under the effects of a polymorph spell, this spell doesn't locate the creature.
I would rule that becoming undead, which changes the creature type, constitutes being in a different form. You would have to have seen the undead form of the creature to be able to locate it.
But, this isn’t super clear, that is, the description gives an example of what constitutes “different form”, but not a definition. So there is room for a DM to rule the other way.