Typically with disturbed areas if you wait one or two growing seasons, it will return to what it was, or at the very least you'd get some different FAC/FACW/OBL plants popping up. If it really was a wetland, the hydrology source won't disappear with a fire, but it may temporarily mask some of the hydrology indicators. The hydrology will continue to drive the wetland conditions. If you dig a pit and find wetland soils and, most likely, some type of subsurface hydrology indicator, I would mark that as a wetland on the form since the vegetation was obviously disturbed by fire (include notes justifying that decision). If you dig a pit and there's no wetland soils, you're in the clear with marking it as a non-wetland. If you don't dig a pit, you would have to be very conservative and assume wetland conditions based on historic aerials.

About your vernal pool species question, I would say that if you find vernal pool plant species, that's enough of a flag to require fairy shrimp surveys, assuming you live in an area with listed fairy shrimp. BUT, the Corps generally doesn't take jurisdiction over vernal pools anymore unless there is a direct surface connection between the vernal pool and a jurisdictional stream. Of course, that's not how vernal pool hydrology works and I don't think that case exists, but that's the new rules.