I've been taking Xanax (.25 mg, 3 times a day) for a while now as I adjust to Lexapro. I haven't experienced a hard time keeping food down like you explained above. I know a few other people who have taken/are taking Xanax as well, and I don't recall any of them having the problem you are having.
Your best bet would be to make an appointment with your physician, explain your symptoms and see what he/she says. A medical professional will be better able to shed some light on your problem. Also, does the vomiting seem to occur after eating a specific kind of food, or does it happen with everything you eat? That's definitely an important thing to take note of and to tell your doctor.
Good luck! Hope this helps. Keep us posted on how you're doing.
It sounds like an esophageal sphincter/motility dysfunction. I recommend making an appointment with a gastroenterologist, or your PCP first if your insurance requires referrals to see specialists.
Please keep me posted.