You get back .5% per hour and it will take you about 8 days to get back to 100%.
You will regen vitality at all times whether adventuring, crafting, online or offline.
Keep in mind that the higher level mob you kill the more vitality will be wiped from you.
When I run through Arc of Harclave in Splitpaaw on Very Difficult setting, all the mobs con yellow, orange, or red. I'll loose about 25% vitality on 1 run.
Even if my vitality is at zero, I'll still go out and do things. I may change what I do to either be 1) run around killing grey mobs for reverse writ drops or 2) run through Harclave again to get max exp possible since I can't go below 0%.
Ever since I started Splitpaw, my vitality has never been above 50%, and is usually in the low teens. Before Splitpaw, I was always above 70%.