I said activatable at will, not useable each round. The fact that a fighter can choose when his or her body restores its vitality (assuming using vitality module), or stamina, assumes that running out of stamina is even possible, without dying. I believe humans drop unconscious before they die of over-exertion. But my point is, why didn't they just make it a reaction to extreme duress. At least this way the DM could keep it in check without resorting to telling that PC not to use their own ability instead of Hit Dice.
It's bad design. SW should have re-used the hit dice mechanic, and then it would elegantly and instantly have been compatible with HD-as-vitality-restoration without any further mucking around.
It's funny that the barbarian actually gets an ability to spark up some rage juice into HP when they become enraged. Right now I see lots of thematic overlapping between fighters and barbarians without any real reason why they have different mechanical underpinnings. (# rages per day, rages allow HD expenditure in some cases, while also granting Temp HP).
When the vitality module is enabled, will Barbarian rage increase vitality? If not, why not? If so, why have a separate HD restoration mechanic, since it will restore vitality instead of healing wounds.
This is the problem with bundling stamina in HP, it makes modules that ostensibly are there to support other playstyles, to expose the underlying structural flaws of merging wounds and stamina into one number in the first place, then giving HP restoration abilities to all kinds of characters whether it makes sense or not.