Keep him on the board long enough, and Vivi becomes a powerful attacker in his own right, but his real value lies in the win conditions you can create with the killer combination of free mana and pinging
The pump activated ability is just icing
Well, I feel like certain archetypes will get some more decent modal spells, which could shore up some of the weaknesses of the more linear spellslinger strategies in Commander
Its adventure can keep your creatures safe for a turn at a fair price, and this cards attack trigger is also great in a deck type that usually goes wide
Wall of Blossoms (Ext, S, Block) The highest placing true cantrip on the list, Wall of Blossoms plays defense with the best of them, without costing you a single card
Manipulating the counters should be easy enough with the commander and various other effects in the deck, so being able to attack or block and getting those cards shouldnt be a problem once the board is decently developed