If you suffer from wrist or elbow pain from using a regular mouse you’re best off doing strength exercises and proper stretching for those muscle groups. I find that to be more effective than using one of these vertical mice as your issue is likely from muscle overuse. Vertical mice can help but if you’re on your computer all day you’re bound to still have the same issue from overuse of your muscles.
That's not how an RSI works. It's not muscle issues at all, it's the tendons that get inflamed from over use and that inflammation pushes against the tendon cords that bind them and that's what causes the pain.