Okay, Snuff marks the first time where Vimes USES the Summoning Dark. But he has not been completely free of its influence since Thud. In Snuff it mentions that he and the SD have crossed each other's paths, usually in dreams, so its reassertion is not a total surprise to him.
And I think I do agree with Tony that Stinky, while not necessarily 'activating' the Summoning Dark, somehow sensed its presence on Vimes. When they first meet, Stinky jumps all over Vimes and keeps on reaching for his left wrist--where the tattoo is, and asking for 'justice.' It's unlikely that Stinky actually visually 'saw' it (one assumes that Vimes doesn't walk around in shortsleeves) but somehow sensed the SD's influence on him.
So, somehow, the SD still exerts its influence over Vimes, of only in a limited way, since Vimes allegedly triumphed over it in Thud! Had Vimes had no absolutely no 'contact' with the SD over the years, then Vimes would have been totally alarmed by its reappearance and tried any way possible to control it. But, somehow, he has moved from being controlled by it to partnering with it.