Is it safe to masterbate after a vaginal birth?

Yes. Your comfort is likely the only guide you will need, but talk to your doctor or midwife about how long you should wait to put anything inside the vagina, based on the specifics of your delivery. Masturbation that doesn't involve putting anything (like fingers or sex toys) inside the vagina is generally safe; if you have stitches or are tender, stop when you have pain.
Sure. Unless you had an episiotomy or tear which has not yet healed. Stimulating clitoris alone should not be a problem. Inserting a vibrator into the vagina should be postponed until everything has healed.