I'm fighting the Kayran and I could only use magic twice (once for Quen to protect and once for Yrden to paralyze the monster so that I can chop it) and then I have to wait for that indicator to fill up and Geralt died before it was filled. Unfortunately, the statue collapsed and I have nowhere to hide. While fighting the monster, that place proved itself an awesome place to hide. Could anyone who suggest me that kind of potion (I've drunk Ostmurk before fighting the monster). Thanks all.

It's in the manual.

Vigor regeneration? Tawny Owl. You can get the formula in Act I.

Also, keep in mind that vigor does not regenerate while you have Quen active; without Quen, it should regenerate rather quickly.

Best strategy is to always keep one bar of Vigor (the indicator below the HP) always available;

1. Use Quen
2. Wait till Quen is dispersed
3. If Vigor = 1, don't use Quen if you are not in danger (wait for another Vigor bar to regenerate)
4. If Vigor > 1, use Quen

That way, you can always have a Quen cast in reserve for emergencies.