I am travelling with my dog from Vancouver BC to Seattle WA. What do I need to cross the Canada/USA border?

All you need is a letter from your vet stating that your dog is in good health and proof of a rabies vaccine.

I don't know if it's still a problem, but many people have had their dog food confiscated at the border because of the mad cow thing. If you do bring your own food, make sure it's in the original package with ingredients listed and that it doesn't contain beef. Otherwise it's probably best to just buy some when you get to the US.

there is not any mountain pass between Seattle and Vancouver. they're the two on the coast. The mountains run parallel to the coast, so, there is not any mountain and consequently no pass. in case you pass inland, there is grant's pass - yet that would not take you close to Seattle or Vancouver.

A LETTER FROM YOUR VET STATING THAT YOUR ANIMAL IS IN GOOD HEALTH AND UP TO DATE WITH ALL OF HIS SHOTS. YOUR VET WILL KNOW WHAT TO WRITE IN THE LETTER.