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Eh, guess I better actually put a argument.....
At full-powered, Dante would dodge around with his ridiculous speed, get close up to Vash and cut the poor guy to ribbons. Normal bullets won't hurt him (or hit him, since Dante has quick enough reactions to catch them....with his teeth), and I doubt Vash will use the Angel Arm thing when they're point blank.
Assuming that Vash begins to use the attack right away, Dante would activate Quicksilver, take a slow walk to Vash, and proceed to brutalize the time-stopped fellow.