Received the cables today and tested the 109xi, works like a charm. Unfortunately the 640x480 resolution still stays a hassle. Only get a clear picture with 800 width and that is not automatically applying due to a bug in VCS I guess (posted in the other thread).
Furthermore I think that about 2 pixels are missing from the width. If I overwrite the 640x480 resolution to 642x480 I get everything, but ofc that turns scaling into more of a problem. If I just change the screen width to adjust, I don't get a sharp picture due to phase.
As it seems like VCS will not receive anymore fixes (until someone else has the will and expertise to continue it) I basically created two compromise solutions now:
640x480 with 800 width screen size. This needs to applied manually. It's missing a few pixels in the width but it results in a clean picture (not sure how exactly as in theory one or two pixels need to be stretched)
640x480 with 801 width screen size. This is basically my "fallback" setting which automatically gets active. Left and right side arent perfectly sharp as the phase isn't fitting. But its better than nothing.
Furthermore I also realized that the signal isn't 100% stable if you touch the cables. These BNC connectors aren't the greatest. I mean usually you don't touch them again, but still (these were new ones).
So overall, the Datapath Vision RGB might be the best capturing device, but it still brings a bunch of problems and is far from perfect as well.