Vandalize is defined as "deliberately destroy or damage". I don't think a food grade powder that will wash away with the next rain counts, but I could imagine some municipalities disagree. Does sidewalk chalk count as vandalism?
Your evidence for it "washing away with the next rain" would be?
Ever see what a child can do with a glass of "cherry" Koolaid?
A dye being labeled as "food grade" has essentially no correlation with its permanence.
> Does sidewalk chalk count as vandalism?
Sidewalks aren't 4,500 year old World Heritage Sites.