There's a Greyhound bus. See links and info on the Festival site. (scroll down to "getting there")
I'm not sure about organizing rides from Vancouver, but it sounds like a great idea. Try a message on the page above. Also, you got me thinking about this, would it be useful to have a page on the site for this purpose? I'm not sure what I could do, but I'm willing to set something up. If I get an e-mail or two about this then I could say on the site that someone is looking to ride share from Van, or Seattle for that matter. Perhaps the Festival Committee needs to have a "ride coordinator" position.