Burgerville closest to I-5?
I don't know how far south you're planning on going, but there is a BV just off I-5 at the Carman exit, south of Portland, just past the junction of I-5 and Hwy 217 near Lake Oswego. Paulj is correct in that the signs on the side of the freeway should be pretty good indicators.
There's one on the east side of I5 at the northern exit to Centralia, WA. I believe this is the northern most one. (818 Harrison Ave)
Typing 'burgerville, vancouver, wa' into google maps, shows 10+, most close to either I5 or I205. I believe there is something like 39 in all.
If you are on the road, cell phone text message to 466453 (google) with a simliar inquiry should give you several locations.