It depends on how bad he is. If he's like a -10 3B defender, just go get an actual 3B and get Vlad's bat into the lineup in a spot where he's not going to hurt you defensively. Unless 3B supply shrinks a lot in the near future, I don't see any reason to use a bad defender there just because it's higher on the defensive spectrum.
Last offseason, legit 3B were not even more expensive than similarly skilled 1B like Alonso and LoMo.