Vray ExtraTex Render Element With Dirt map In texture slot causes progressive rendering to hang on pass 0
Scene please?
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VRayDirt subdivs are set to 512 and that’s the cause. Why have you set them that high?
I thought higher the number the nicer the AO would be. what setting is recommended?
Depends on your scene, but still 512 seems unnecessary high. Try with lower values first (the default as a start) and if you are not satisfied with the result then raise the subdivs.
The AO map is sampled those many subdivisions for each camera ray.
The default value of 3 subdivs will shoot 9 AO rays per camera ray, and when you reached the maximum default sampling (100 subdivs) it will have cast at most 9 x 100 x 100 = 90.000 AO rays in total.
This, only in the unlikely event that the AO won’t have cleaned to noise threshold well before that value.
512 subdivs is 512 x 512 = 262.144 rays, for each camera ray.
Because of how V-Ray works, those rays will *all* have to be cast first, and then V-Ray can check what the noise level is.
So it’d take 3 times the max samples than the default, on the first pass only.
It’d then likely stop sampling it, but the damage by then would be done.
If you had a min AA higher than one, it’d cast all those rays more times than once, compounding the endless wait. 
if you have many AO maps around a scene, i can give you a little one-liner script to reset them all to default subdivisions (paste into the maxscript listener and press the keypad enter):
for ao in (getClassinstances vraydirt) do ao.subdivs = 3