I doubt it would void the warranty for hardware purposes. If you have a bad charging port or the like, I don't see how they could claim it was from unlocking/rooting, but you never know. VM has been pretty good about that sort of thing in the past, so I wouldn't count on them saying it was voided because of that.
If there is something software related (i.e. you bricked your phone) they would probably saw that wasn't under warranty.