Tony, Michael Woolfe has detailed a route up thru Vancouver to Longview:
from Longview, hwy 411 up to Vader's is not bad as i recall - and i don't think there's another realistic option.
From Vader, Hwy 506 west to Ryderwood & continuing north thru Boisfort & Curtis is a really nice ride - & almost no traffic as i recall. you end up on Hwy 6 about 8 miles west of Chehalis. From here, it looks like there's a paved road north to Rochester: Bunker Creek Rd. / Ingalls Rd. / Independence Rd. But I've never ridden it, and the Benchmark Atlas is a little unreliable at times on their paved/unpaved distinction.
There is a good back way over to the Rochester on the south bank of the Chehalis.
ok, enjoy (and hydrate)!