You have it right...cross border at 15 (pacific hwy) and then take 1 to Capilano Rd exit. ....99 will dump you right into downtown Vancouver streets with no freeway and the 1st Narrows bridge with 3 narrow lanes and variable lane control. Not at all where you want to be with your rig!
If you are not intimidated driving in close quarters traffic then stay on I-5 thru Portland if off peak traffic hrs, otherwise the 205 but it is 20 miles longer.
I-5 Seattle traffic just is what it is for 30 miles north or south of downtown but on I-5 computerized overhead lane speed control signs slow traffic when it is conjested so driving bumper to bumper in tight lanes is not too stressful.
There are rush hours in Seattle and Vancouver, but no rime or reason to conjestion some days. Sit back, relax, go with the flow...Vancouver traffic listen to AM radio1130 or 730.