If you steal Vindicta's snipe as Sinclair and get a kill, it gives her a stack

[Vamptoria]

Found this in game and thought I had to be mistaken, so I tested. At the start of the video, Vindicta has 0 stacks, then after Sinclair gets an ult kill, it goes up 1 (+10%)
View attachment MedalTVDeadlock20260317165143177.mp4

 

[VictorKain]

Aie, well not a good idea to steal her ult for now if she gets stacks.