What would have happened if Romney had vetoed the bill?
It's irrelevent. He had a sworn duty to uphold the Constitution. As usual, he failed in that duty.
The overwhelmingly Dem legislature would have overridden his veto and Mass. gun legislation would have been far more restrictive than the compromise bill which Romney was able to work out with the Dems.
Romney learned and lived the Reagan lesson:
"I'd learned while negotiating union contracts that you seldom got everything you asked for. And I agreed with FDR, who said in 1933: 'I have no expectations of making a hit every time I come to bat. What I seek is the highest possible batting average."
"If you got seventy-five or eighty percent of what you were asking for, I say, you take it and fight for the rest later, and that's what I told these radical conservatives who never got used to it.
~~ Ronald Reagan, in his autobiography, An American Life .