If one quart of veggie oil was out into a trucks transmission system and actually sinks in tranny fluid. Where would it settle in the truck?

in the lowest point of the transmission sump

Vegetable oil FLOATS in transmission oil.

possibly the trans pan or torque converter... probably be all over, only reasonable solution is drain and fill, possibly two times.

Pull the tranny pan and replace the ATF filter while draining the system. Who the hell put veggie oil in a truck? If the truck was started you'll need to flush the transmission after replacing the ATF filter so that the veggie oil is removed from the torque converter too.