In the game's current state, Vigor is perhaps the least used shiftstone, often getting discarded for adamant and stubborn for damage negation effects.

Besides just making it heal more or requiring less damage (I'm gonna out some thoughts to these at the bottom, but they're not the main focus of this post), I think a good change would be to allow the healing to go over 20 HP.

A problem that all defensive stones share is that they don't really do anything until you get hit. Stubborn, Adamant and Guard however at least interact with that first hit and reduce the damage.

Vigor already has the downside of having a limited amount of healing to give each match (6 if the opponent isn't playing damage negation), so in some scenarios getting a 5 damage burst while staying at full HP - something that should feel great - can feel like you just lost 2 HP you could have healed to gain even greater HP advantage.

On Changing the Numbers:

I feel like a 3hp heal would be justifiable for how hard it is to proc, 4 would maybe even be too much.

I'm against lowering the activation number, since that would completely change its current playstyle of going for two hits and follow ups to fishing for singular, strong hits with Walls and Grounded Cubes.