Vigour is spent continuously — by running, by working a station, by channelling — and it returns on its own when you rest. It is one budget for the whole body: the same pool is your stamina and your mana, so how you arrived somewhere changes what you can do when you get there.
Running out
At zero vigour you fall prone. You are incapacitated, not unconscious — nobody has to revive you. Once the pool creeps back over a small threshold you can stand and carry on.
Reading the pool
The strain you feel is deliberately back-loaded:
strain = (1 − vigour/max)². The top of the pool drains almost
invisibly; the world only starts dimming at the edges in the last fifth,
which is where your decisions actually live.
What it will not do
Vigour does not regenerate while you are working — a body at a bench is spending, not resting. And going hungry or thirsty drains vigour rather than health: starvation drops you, it does not kill you outright.