Duskthorn

It's actually +20 Str, to a total of 45 by lvl 25.

Hi Vigor is unlocked at lvl18, thus can only receive 1 point at lvl18, regardless of how many spare skills points you have. And yes, 1 point each in Defiance, Prayer and Cleansing is required to access Vigor. At lvl19, you can put 1 more point in Vigor, and so on.