(Wait, do you have to use source vampirism on living targets? I usually sucked on ghosts or corpses.)
I think so. That would make sense.
Stealing a Source point from a living character is essentially murder.
The relevant script is named
GLO_SystemicTags, which determines when the HERO and VILLAIN tags are added, as well as some other related tags. The whole script revolves around murder.
There are three base Murder Types shared by default across adventures:
1. Murder
2. SneakMurder
3. SummonMurder (which may not cause the VILLAIN tag to be added, by the way)
Because Source Vampirism is unique to the Origins adventure, Larian had to implement Source Vampirism as a fourth custom Murder Type in this script.
I haven't done any scripting in this engine, but you can read my inline comments in
this snippet, if you want to get an idea about how all this works.