It could be viable but there's no recovery for him at all since he has the Assault Vest.
You're better off making him mega.
Trait: Regenerator --> Shell Armor
EVs: 252 Hp, 156 Def, 100 SpD
- Calm Mind; The move that will basically turn Slowbro into a tank
- Slack Off; Reliable recovery
- Scald; STAB, can make your defense even better(if physical sweepers and tanks are burned)
- Fire Blast/Psyshock; Cover your grass and bug weaknesses with Fire Blast or get STAB with Psyshock
Hope I helped!