Is a wingtip vortex strongest at the critical angle of attack? Generally, wingtip vortices increase in strength as the angle of attack increases.  Is the critical angle of attack/Cl Max the peak strength, or do the vortices generally continue to increase in intensity once the plane is in a stalled condition? For the purpose of this question, assume the stall is not an accelerated stall and is entered from straight-and-level flight in smooth air.
If taken to an extreme, such as a 90 degree angle of attack, there would no longer be wingtip vortices, just turbulent flow and drag.
If it doesn't peak at the critical AOA, where does it peak?
do the vortices generally continue to increase in intensity once the plane is in a stalled condition? For the purpose of this question, assume straight and level flight in smooth air

These sentences are contradictory. Never mind.
The strength of the tip vortices post-stall depends on the wing's stalling characteristics. If the inboard section stalls first (which is desirable because it helps keep the ailerons functional) then the tip vortices will continue to increase with angle of attack until the tip section itself stalls. At that point lift near the tips drops sharply and the vortices will decrease accordingly.
If the plane is capable of post-stall flight at extreme AoA, it will not be travelling fast enough for the vortices to maintain significant energy.