I don't know if using the flavor extract would make a difference, but if you used the actual ingredients (vanilla, maple syrup) that would go into Spice Herb Veg (30 A) so based on the flavor profile that's where it would fit, but I think this might be the first time I see one that's an honest to God Experimental Beer (34A). It doesn't *exactly* fit into SHV because you aren't using actual spices.
"As the number of Specialty-type categories is quite large, most beers are expected to be entered somewhere else unless there is something quite unusual or unique about them."
Sounds like you might have something unique. Since it's a generic catch all category you'll have to be very explicit in what you tell the judges.. something like "American Porter (20A) flavored with French Toast extract, evoking flavors of french toast batter, vanilla and maple syrup."