You might try slowing down her drinking by adding a couple big clean rocks to the water bowl so she has to lick slowly. Or fill the bowl with icecubes and let her drink the melting water. You can also add extra water to her dog food and see if she can hold that down so she gets the fluids she needs.
It is possible, with her age dog, that she has swallowed a non food item that isn't passing through her system and doesn't show well on x-ray and is 'sloshing' around when she drinks triggering the vomiting. With my dog it was a latex squeaky toy swallowed whole and it would 'bang around' when her stomach was empty or when she drank a lot of water at once making stomach contents really liquid. Once the toy was out the problem was resolved.
If you haven't tried a barium x-ray to check how her swallowing goes and to see if anything shows up that regular x-ray misses that might be an option as might be using an ultrasound to check the same areas.
This could be a problem starting with megaesophagus which you can read about here
http://www.marvistavet.com/html/megaesophagus.html
again some more testing might show you if that is the issue.
If your vet is stumped you might ask for a referral to a small animal internal medicine specialist as those see more of the tricky cases than a general vet might.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics