I have not run into a provider that did NOT have a vibrator (or five or six...). I don't know if that is common (my sample size is way too small) - the providers I book are mostly sex-positive, more mature women. I don't book a second time with one that is not sex positive. The ones I see on a regular basis are really pretty open about sex and what works (and doesnt).
Some like really the Hitachi- some like something a little less like a bullet. So - rather than bring her one, probably best if you use hers if you are using it on her. Key rule with this - Pay freaking attention to what she tells you.... a good rule to live by in or out of P4P...
You can use the Hitachi with your fingers between the vibe and the vulva as a pad so things don't get too intense. Remember to start slow, caress and don't attack her clit like a demented weasel. In fact, until she lets you know, stay the heck away from it. Lube is also your friend... take your time. She'll let you know when you need to get there... Do it right and you will get a marvelous response... or a really good acting job - and honestly, as far as you are concerned, why would it matter... As participatory performance art goes, it's really a lot of fun.
If you are using it on you - and you have seen her before - ask her before your next session. It can be a lot of fun - BUT you might want to try it on yourself first just so you know what's good on what part... taint, testicles, frenulum, corona, shaft whatever - and let her know. If she does it right, you'll hit the ceiling (in a good way..). It turns out that you can, in fact, ejaculate without an erection just using a vibrator... So if junior ain't cooperating in the second or third round - there is hope..