Can mild bladder or vaginal prolapse be corrected with kegals?

Yes: Pelvic floor muscle exercises can be very effective if done on a consistent basis with many, many repetitions per day. Sometimes, specialized physical therapy provides very effective training in how to do these exercises properly.

Pelvic muscles: Although prolapse might not be reduced by increasing vaginal/pelvic muscle strength, you may be able to limit its progression. The most important time to "squeeze" is before you cough or sneeze as the force will be distributed across the muscles and not directly to your weak spots where things are prolapsing.