Burgerville closest to I-5?
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.
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.