What and where are good (fast and easy) sources of Vials of Weak Blood
Solution 1:
The river outside of the Township of Claypool is infested with brackish skale, and they drop vials of weak blood as their fine crafting material. The further you go downriver, the more dense they are.
Other foes that drop this item include minotaurs, harpies, bats, spiders, skelk, ridgeback skale, and bog skale - see this relevant wiki page.