My first thought was something that doesn't show up on xrays. I heard of a dog who had swallowed the plastic squeaker from a toy, and though it didn't show up, they saw a mass...they did surgery & found the squeaker lodged in the stomach wall (I think) and the tissue was growing around it.
My second thought was irritable bowel disease, but I don't think that comes on this suddenly.
I just did a quick search on "chronic vomiting" and found this site:
http://www.wellvet.com/vomit.html
Keeping my fingers crossed for your pupper!