So I read somewhere that NPCs have a chance of becoming infected with the vampire disease if they fight a vampire in an attack. They will actually gain the appearance of a vampire as well, afterwards. (I'm not talking about the Volkihar quest that allows you to turn your spouse)

Has anyone actually had this happen?

NPCs can't be infected with Vampirism.
The only way to turn an NPC is by doing the Volkihar quest.

This. Even at which time their appearance doesnt change. At least it didn't for me.