With the release of V-Ray Next, development on V-Ray 3 has ended. Due to this, we cannot offer a backward-compatible solution in the form of a v3 build.
Regarding the comparison with v2, be advised V-Ray for Rhino has been pretty much rewritten for v3 and v4 to ensure no legacy code can hold back its future development and to allow Rhino users to benefit from the same advancements and new features that our other flagship products receive. With changes of such scale we understand some new bugs were introduced and for this reason focused on improving stability with every update ever since.
In the case of SaveRenderWindowAs - it has been affected by 3 separate issues which have all been resolved. Only one of them, however, made it in v3.6, the rest were fixed in time for the v4.00.01 release.
That said, if you experience any other severe or recurring issues, please notify our Support team to forward your correspondence to me so I can look into them as well! Stumbling over various problems almost every week does sound uncommon and also very troubling.
Finally, if you wish to check whether the latest V-Ray for Rhino meets your standards I recommend downloading its trial version (its functionality is not limited in any way) or requesting an evaluation license by emailing firstname.lastname@example.org