Hopefully I'm not too late to the party, but I've done this!! Was in exactly the same situation as your lady -- I was into it mentally because of how much of a turn-on it was for him but the physical aspect was a bit strange at first... So (as a biology major) decided it would be a fun experiment.
I don't know if this works for everyone so easily, but worked for me within a month or so. I'm usually quite hard to get off without the vibrator so, (strictly speaking, in terms of conditioning) the 'reward' was of greater value to me than it would've if I were getting off loads without it too. I would recommend investing in a really good, powerful vibrator -- the sensation of having something in your butt can be a little harder to get past and reach orgasm than she finds usually. Definitely worth spending a bit more and making sure it's going to work. Think of it as an investment.
Also, start easy. Don't destroy try and go to pound town early on because it will put her off for life. If she has a bad experience along the way then you'll do more damage than good (both mentally and physically). Again, think of these early steps as laying the foundations and don't get greedy. It might help to have her control the tempo/depth of penetration initially while she gets used to it, for example.
In the same way that if you're training an animal (sorry if that's offensive), it's important to keep the reward high even if she does reach the point of being conditioned. Make sure you keep up with a 1:1 reward ratio to start with (she cums every time you're in her butt). It might drop with time, but it's still important to maintain a reasonably high reward ratio. Without being rewarded every once and a while, a phenomenon called 'extinction' tends to occur.
Also, as others have said, I'd probably recommend you fill her in on this. Personally, I don't think it's really fucked up/manipulative but best to be open about these things at any rate in case she finds out in the future and is offended you've treated her like an experiment.