Image: Rebel Wolves / Bandai NamcoNo screenshot of Vrakhiri Blood Phials yet. This one shows Svartrau Cathedral (Rebel Wolves / Bandai Namco).
Vrakhiri blood phials hold the blood of a named vampire. Drinking one unlocks that vampire's power for Coen, and later sips upgrade it, so they are the key to most Vampirism abilities.
Phials come from quests, camps and hidden caches. Examples include Xanthe's Recruiters: Taste of Power for Brencis's blood and the cathedral treasury for Xanthe's.
Pick the option to collect the blood when a quest offers it, or drink from a vampire you defeat. Get Xanthe's cathedral phial before you settle her fate so no choice is locked out.
The first drink opens the ability in the Vampirism tree. You still learn it at a road shrine with a skill point and enough Corruption.