Let's say we have to go into Iran because there is firsthand intel that shows the mullahs to be just months from getting the bomb and we go to the UNSC for a vote and China and Russia veto. But our leaders, hopefully, decide that our national security is too important and attack/invade Iran anyhow to neutralize the threat. If we act contrary to the UNSC permanent veto holding members then who is to say they won't do the same sometime in the future when we try to use our veto?
Well, they can act on their own; our veto only applies to the UN taking collective action. Every country still can take whatever action it deems fit on its own as a sovereign entity.