Whoa. If you are cutting off parts of the subject with the 15mm then you are REALLY close. (which is kind of a good thing with such a wide angle lens)
I also use a Canon 1dsmk2 but was fortunate to have a lot of rangefinder experience years ago. Using a RF requires very good "camera technique" that can take a while to learn but be patient, it will come.
1. You are using a 21mm external viewfinder on the RD1s, not the 15mm that comes with the lens.
2. If you are real close to your subject, use the dotted lines (parallex compensation) on the top of the viewfinder for framing, not the solid line.
3. You don't mention having a focus problem but be sure to remember the hyperfocal distance crop factor too.
That lens has such a large dof, you should be able to preset your focus for most situations, then frame carefully through the external viewfinder then snap away.
I also use the 15mm a lot on the rd1s and aside from the vignetting, it is a wonderful lens especially for the money.
Hope this helps,