The #6 does indeed go all the way to Vancouver. The end of the line for #6 is the 7th Street Transit Center which is in downtown Vancouver and is one block away from the hotel.
I used to live in Vancouver, I've ridden it many times, and the #5 before that. Here is a sample of the directions that Trimet will give you.
1. Walk southeast from Merlo/158th MAX Station to Merlo/SW 158th Ave MAX Station (Stop ID 9828)
2. 11:22 a.m. Board MAX Blue Line to Gresham
3. 11:44 a.m. Get off at Goose Hollow/SW Jefferson St MAX Station
4. Walk southeast to SW 18th & Columbia (Stop ID 1114)
5. 11:59 a.m. Board 6 M L King Jr to Vancouver Washington
6. 12:50 p.m. Get off at Vancouver TC & 7th and Broadway
7. Walk east to Vancouver Transit Center
Travel time: 91 minutes (including 3 minutes walking and 15 minutes waiting)
#6 starts at the Goose Hollow MAX station, so you could equally transfer from MAX to #6 at Goose Hollow or at the Oregon Convention Center MAX station. I would recommend transferring at Goose Hollow because it takes the MAX so long to get from one side of downtown to the other.
