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First - see if you can use a 2" nosepiece instead of the 1.25". That should help with the vignetting. Flat frames can help with that too.
RC scopes don't have a 100% flat field, so stars at the edges will look a bit elongated unless you get a field flattener. But even with a field flattener, it's challenging to get everthing all lined up perfectly so the optical axis / collimation is dead center & symerical - as you noiced with your images, even though you are not too far off.
One thing you can do is point to a bright star and go in live view mode and put the star in the center, then each corner and zoom in on it and compare the shapes. Do the same focused and defocused - inside and outside focus. You can play around with the collimation while you do that to get an idea how easy/hard it going to be to get it to a point where you are happy with the image.
Or you can do what Leo said and leave as it stands!
James
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