I just joined Hubpages today so I'm not sure how these questions usually get answered, so I hope I don't commit any faux pas.
From a strictly bone healing point of view, by six months your bones (tib/fib) should be strong (usually very strong by 6-12 weeks unless there are complications). By six months I would expect that you'd be getting back into whatever activities you did before the fracture. Without seeing your x-rays, taking your history, knowing details of the plates/screws etc. it would be inappropriate for me to offer specific advice about this, but a session with a physical therapist might be a good idea to put together a comprehensive rehab program that you can undertake on your own, to get back to whatever exercise you used to do / want to do. You can probably pick one with access to a vibration machine if you are interested in giving that a try.
I've seen many people not reach their full potential after ankle fractures simply because they never really tried to get advice / help (and / or didn't put any effort in to rehab exercises) for their ankle. So while it may well handle using a vibration machine (can start on low settings), it's probably best to get specific advice from your Dr. or a physical therapist about the best overall program for your ankle to get your exercise ability back to where you want to be.