Not sure if this is much of an issue, but I just managed to crash MAX:
1. Open old scene (pre VRay 3.3) with active render region
2. Change render size, so the region is out of bounds.
3. Set Progressive Image Sampler
4. Hit Render → Crash
No Crash with Adaptive Sampler, no crash when the region is disabled or a new region is defined.
Stripped down scene:
vray_progressive_region_debug.zip (2.82 MB)