We usually see vomiting coupled with this crazy grass eating.
Lots of dog eat grass and some dogs vomit routinely after eating it. In these dogs, it sometimes seems to help meet the urge for green things but cause less vomiting if you give them green beans (and they will eat them). Often, we can not really find a cause for the grass eating but sometimes the vomiting is actually a sign of a problem like inflammatory bowel disease or even internal parasites. If this occurs at times when your dog is not eating grass it would be good to schedule a vet visit to see if the problem can be resolved.
Many people think that dogs eat grass just so that they CAN vomit (if they have an upset stomach) but this has never been proven.
Let me know if you still have questions.