Hmmm let's think....
Featwise Vash hasn't done as much as you would expect (this is a gun related anime/manga/comic after all) but the angel arm itself blew the hole through one of gunsmokes three moons (I think theirs 3 ?) at 3% power.
Also the blast on the city of july didn't kill everyone inside it just destroyed the city , the people inside starved to death afterwards.
The highest the Angel Arms has ever gone is 23% i think, but that drained a chunk of vashes life-force (Shown in the manga as a black streak through vashes hair)
Needless to say though vash is a good enough marksmen when provoked so he should be able to tag slade with relative ease , possibly knee capping him and then he'd go home , cry over ruining slades life and eat some donut's
vash should win on paaper, but slade would pull a victory...and it doesnt have anything to do with speed or reflxes....remember vash has a strict code not kill, more strict that superman and rourini kenshin combined....so yeah he might do some trick shooting like he alwasy does and you see slade's weapons unraveling in his hands but a few mortal wounds here and there and vash would bleed out..cuz we have seen that he has unecessarily put his life at risk and incured massive injures just so that he can avoid taking a life....but if this is a ballsout matchup, without consequences vash could do it with reg. shooting alone, but if slade's healing prove to be too annoying its atomic city baby!