I use Thomthom’s Vray tools and Vray day to day, but usually don’t adjust the frame ratio. I typically just render what is shown on my screen and adjust later in post.
All in all, I think this is an expected behavior.
According to a previous thread:
The FOV, field of view is measured vertically. (I personally don’t think this is how it should be. but that’s just how it is)
Therefore, when you change the frame ratio, you are effectively changing your field of view since you are reducing your frame height.
Does that make sense?
Why not adjust your image in post after render?