You should just evolve it. It's a rare case when Slaking Won't OHKO something with a decent attack. It's got better all around stats and even if it has Traunt, (I think) Slaking's base attack divided by 2 is better than Vigoroth's.
If you want a water Killer, Magneton is good. ^_^. Regice is also good, because of it's insane Spdef. and decent Spatk.