Well, Slacking is one of the strongest Pokemon around, I mean, it literally is stronger then some legendaries. The problem is, its ability pretty much cuts all its stats in half, depending on how you look at it, and Vigoroth doesn't have that ability. In my opinion, neither learn any very good moves, except Slaking's hammer arm, but thats just me. Overall, neither is a very good fighter, but for filling your pokedex, go with Slacking.