I have a 9yr old Chocolate Lab/weimeraner?

If he is not showing any signs now and the vet was not able to do anything prior, but prescribe medication that you have not continued to keep him on anyway than there is no use taking him back to the vet until there is more signs of something else going on. For instance, you said that he is drinking, is he eating? I'm pretty sure that he is if you did not mention it in your question. He should be put on a bland diet, like boiled chicken and rice, for now until the stomach has time to stop being upset, or either have food withheld for a little while. You should make sure that he has plenty of fresh water supplied and limit his time outside, and definitely monitor it to make sure that he does not ingest any more weeds that contain the roundup. This chemical is very harmful to even humans if ingested, so he should not be given an opportunity to ingest any of this. At least, do not let him gain access to these areas that have been treated for a couple of days. Only treat areas that he does not use, or treat some areas and have him use another area for a while why this area has a chance to become less hazardous to him. Yes, I believe that the roundup may have caused this episode for you dog. Good luck, and remember bland diet until his stomach feels better. Also, you may want to look at the roundup container and look for the poison control number, you can call this to find out about other things you can do to find out if these are signs of ingesting the product, or to relieve his symptoms.