I had a pup that had just left and they took her in for her vaccine andrabies, she did this. She was still at the vet but had to have IV's benadryl and more. She pulled through but I did send all my pup owners in the litter an email cautioning them since the one pup had a reaction. I now always have liquid benadryl on hand. No one could say why this happened but my vet cautioned that it could likely happen again to the same dog. I also know of someone with Doxies that splits up her dogs' shots.
Good luck and we're sending healing thoughts your way for a speedy recovery!!!!!!
So scary! Hope Gracie recovers quickly. Some dogs can have bad reactions to vaccines, just as some people have bad reactions to certain medications. DHPP is a core vaccine and Parvo particularly is frequently deadly if not vaccinated against, but you can talk to your vet about future shots once the crisis is resolved.
Keep us posted!