To flap or not to flap, that's the question for Mr Vortex
I fly simple aircraft such as gliders and L-19 towplanes.
It is my experience that more lift needed to be provided by slower airflow, using a clean wing, is one source for vortex increase at the tips.
When lowering flaps, the wing's AOA decreases at a given airspeed, hence the wings are at less AOA producing less vortex. However, the airflow around the flap tips will now become the large vortex generators.
I follow other L-19's closely on final and have experience strong roll tendencies of my aircraft when flying through another L-19's vortex.( created by its use of full landing flap.)
Mind; the lowering of flaps at a given airspeed and AOA requires an increase in power setting to overcome the added drag OR the aircraft nose needs to be lowered to overcome the added drag i.e. steepen the glide path, which is exactly what the pilot needs to land in a given space at the slowest safe speed.
I am not very scientific with this explanation, but it helps me understand the operation of aircraft.