How dangerous is aspirin for my dog?

Your only real mistake was the baking soda. Your dog suffers more from the sodium than the asprin. To the that claims to be a vet, what school did you attend? Asprin is one of the NSAIDs, Nonsteroidal ANTI-INFLAMMATORY Drugs that can be used relaively safely by dogs. It is best to have it in buffered form, and give it wiht food. You'd have to ingest a lot of asprin over a long period to kill a dog via internal bleeding with it.