I can’t understand why the vet would tell you not to use meadow hay. If you were feeding straw then the vet would be right in telling you not to use this because as far as I know there isn’t much nutritional value in straw. But meadow hay is fine. I’m from the uk and I have occasional used wilkosTimothy hay which my bunnies seem to like so you could see if your rabbit likes this aswell to add a bit of variety. I buy hay from b and m and I use both meadow hay and Timothy hay. I don’t use anything particularly expensive and my rabbits still eat lots of it. If your rabbit has healthy poo I can’t see any cause for concern. I personally like to use cheap meadow hay as I find it a lot coarser and by giving them different varieties and textures to chew on it gives their teeth a good wearing down in different ways.

Maybe the vet just thought it was straw if she saw a particularly chunky piece of stalk? Meadow hay is very common here, most hay that’s sold here is meadow. But you can of course try some timothy in between, for variation.