It's definitely do-able! I ride the opposite way every day. I live in Vancouver and work in Sellwood.
I usually follow Interstate ave. going north. Others take Williams/Vancouver. Here's my route. I hope it helps:
Take the esplande up to the Rose Quarter.
Follow Interstate ave north.
Cut over to Denver at Alberta or N Willamette Blvd.
Follow Denver until it crosses back to Interstate. Take a Left.
Follow interstate down to Victory Blvd. Take a right.
After the first light on Victory, take a left into Delta Park.
Follow that road to the end, cross the street (an exit off of I-5), and take a left onto the sidewalk/path that leads up through Jantzen beach.
The route through Jantzen beach is difficult to describe, just head for the bridge.
The bike/ped path on the bridge sucks. You get used to it.
You'll end up on Columbia Ave. in Vancouver. Go north to McLaughlin (almost a mile). Take a right. You don't want to bike on Fourth Plain.
I'm not too familiar with the best roads after that. It looks like you might cross over to 18th after McLaughlin crosses Grand. You might also opt to take the #4 bus from downtown Vancouver. It goes down Fourth Plain.
Also, take a look at the #105 (C-Tran). It runs between downtown Vancouver and downtown Portland.