Let's take a look:
[Vigoroth][1]
HP 80
ATTACK 80
DEFENSE 80
SP. ATK 55
SP. DEF 55
SPEED 90
[Slaking][2]
HP 150
ATTACK 160
DEFENSE 100
SP. ATK 95
SP. DEF 65
SPEED 100
Judging by this alone, Slacking is miles better. They learn the same moves, give and take a few. I suggest evolving and in double and triple battles have a psychic Pokemon use skill swap to remove Truant and let Slaking reak havoc. If you're lazy and don't want to do that EV train till the Miltanks come home.
Fell free to comment for anything. V78 :)
[1]: http://pokemondb.net/pokedex/vigoroth
[2]: http://pokemondb.net/pokedex/slaking
