Okay, so just a few frames after the screenshot where I got my last vector, I spotted baby Spike making this epic face at Twilight's sudden epic magical explosion. Naturally, the decision to vector it followed quickly after.
I suppose it is a little odd how his mouth doesn't have that particular stroke, but his body does. That's how he looks in my references, although I think that particular stroke is supposed to be a little thinner, and therefore more discreet.