Because by not getting a deal he made Trump look bad - and Trump would thus fire him(in the US to throw someone under the bus means to assign blame and punish someone else)

But Vance is the vice president and Trump can't fire him? He is not Secretary of State.

And I really don't think Trump looks worse than before. If they reached an agreement that allows Iran to take tolls for all the tankers passing, that would look really bad.