The MVP voting system is supposed to be for DF parties only. You will not be able to vote for someone you queued with, only for those DF randomly matched you with. While I am sure people will eventually find ways to abuse the system it's not going to be as easy as just voting for your buddies, seeing as how chances of you being randomly queued up with then are nill.

It's really not a bad idea, provided the voting is done AFTER the run is over (and strictly voluntary of course), so that tanks/healers can't coerce the rest to vote for them or else.