I broke my back 8 months ago and my menstrual period has not yet returned, is that normal?

Yes, it may take up to one year for your period to resume after the injury.

I am a man with spinal cord injury, is there anything I can do on my own to improve the quality of my sperm?

Unfortunately not.

How often can I use the vibrator?

There are no limitations.  Stop if your penis becomes red or if you have a persistent erection.