I can't figure out how to get Weak Auras to notify me when the PetVehicle is casting a (specific) spell. Does anyone know how to do this? It's not on the dropdown for "Unit".

(The spell is "Vine Slam" and is cast by the wild vines in the Sunsong Ranch. It took me quite a bit of digging to figure out that the wild vine is counted as a "PetVehicle." I tried all the "Unit" choices and a few other ideas. HELP ME!)