DEFINATELY make the Vigoroth evolve! (Vigoroth is actually a competitive Pokemon, but Slaking is better).
Why?
Look at this.
HP
Rayquaza 105
Lugia 106
Ho-oh 106
Mewtwo 106
Groudon 106
Kyogre 106
Slaking 150
Slaking's HP is higher than the HP of all those ubers!
Attack
Rayquaza 150
Lugia 90
Ho-oh 130
Mewtwo 110
Groudon 150
Kyogre 100
Slaking 160
Slaking has a higher Attack than all those ubers. This means he will deal more damage than even a Groudon and Rayquaza! Still dont think its good? Its won two categories already against ubers...
Defense
Rayquaza 90
Lugia 130
Ho-oh 90
Mewtwo 90
Groudon 140
Kyogre 90
Slaking 100
Look at that. A Slaking will even last longer than Rayquaza, Ho-oh, Mewtwo and Kyogre. Not only that, but it has a higher HP meaning that it will still last almost as long as a Groudon and Lugia in terms of physical attacks.
Special Attack
Rayquaza 150
Lugia 90
Ho-oh 110
Mewtwo 154
Groudon 100
Kyogre 150
Slaking 95
This is one of Slaking's weaker points. But think about it. If Slaking deals more damage than a Lugia it is surely worth using, even as a Special Attacker. Not to mention its special attack is only base 5 lower than Mew's, Celebi's, Jirachi's, Starmie's, Groudon's, etc.
Special Defense
Rayquaza 90
Lugia 154
Ho-oh 154
Mewtwo 90
Groudon 90
Kyogre 140
Slaking 65
Okay, Slaking's Special Defense does not look that good. Think about it though. It has no special weaknesses at all! None! (and incidentally, only 1 physical weakness). Not to mention, Slaking's higher HP means that he still lasts almost as long Groudon, Mewtwo and Rayquaza at the end of the day.
Speed
Rayquaza 95
Lugia 110
Ho-oh 90
Mewtwo 130
Groudon 90
Kyogre 90
Slaking 100
What does this mean? It means Slaking is faster than Groudon, Kyogre, Ho-oh and even Rayquaza! And if a Lugia has low speed IVs, EVs and a bad nature for speed, you may even be able to outspeed with with your Jolly, Timid, etc. Slaking!
Stat Totals (the biggie)
Rayquaza 680
Lugia 680
Ho-oh 680
Mewtwo 680
Groudon 670
Kyogre 670
Slaking 670
Slaking has the second highest stat total in the game! (=5th highest if you count ties). If Kyogre missed every other turn, would you use it? Slaking is definately worth using. It is one of my favorite Pokemon. And, if youve ever heard of the Skill Swap truant combo, you will knoe why Slaking is considered good. Without Truant, it will be the strongest Pokemon allowed in the Battle Frontier and Tower. WIth Truant, it is still one of the stongest Pokemon.
Yeah, sure, Vigoroth is good and competitive, but when you have a chance to get a Slaking, why miss it?