For solo play, a
condom is not necessary unless you really want to make
clean up easy. If you are sharing your vibrator with a sex partner (or
partners), put a condom over it and change the condom each time it is used
by a different person. In either case, if you are inserting the vibrator
both vaginally and anally, use a condom on the vibrator and change it
between orifices to avoid infection.
Long Answer: Putting a
condom over your vibrator is a good way to extend its life an make clean
up easier, but it isn't necessary for solo masturbation unless you're
using the vibrator in both your vagina and ass. You should never put
anything in your pussy after it's been in your ass, so if you're using
vibrator anally, cover it with a condom first, then
remove the condom before you insert it in your vagina or touch it to your genitals.
If you're sharing a
vibrator with a partner, you should definitely cover it with a condom
and change the condom each time the vibrator is used by a different
person. This helps avoid mingling sexual fluids and transmitting STDs or
microorganisms that can cause vaginal infections.