A fiscal union would have transfers from various parts of the union to various other parts over the business cycle. A guarantee to stand behind somebody’s debt while simultaneously insisting that you’ll never actually need to cough up a cent because you’ve made them pinky swear is not a fiscal union.
A veto stops a thing from happening (think of the UN Security Council). The fiscal compact is going to go ahead, just without Britain. Therefore, Britain did not veto it; they declined to take part.
That is all.
Update:
Okay, that isn’t quite all. Just to be clear, I think that Cameron did the wrong thing. I believe that, at a minimum, he should have committed to bringing the proposal to the UK parliament. It may well have been voted down at that point, but nevertheless it should have happened. Parliament is sovereign in the UK. This was a serious proposal with potentially significant consequences from either agreeing to it or walking away from it; the people of Britain deserved to have their elected representatives decide.
I am undecided on whether signing up to the pact would be in the best interests of the UK.