The last time there was a roll call vote for the VP slot was in 1976. I think the non-Romney (formerly anti-Romney) contingent could make a roll call vote a precondition for releasing their delegates. They could tell Romney to offer his choice of a running mate, along with a few alternatives put forth by other people/factions in the party. The conservatives would see it as giving themselves more influence over the ticket, and could conceivably do so if they picked one person and stuck with them. Romney would probably end up getting the pick he wants since there would probably be an insane amount of scattering of votes for other candidates.
Only if (1) Mitt Romney will do so as a sign of Party unity, or (2) the Presidential election is basically lost as in 1964, 1972, or 1984.