Personally I would take 9 to 542, then 542 to Bellingham and pick up I-5 South then I-405 to I-90, then I-80 to I-82, I-82 south to I-84, I-84 to Twin Falls, ID then US 93 south to Ely, NV, then US 6 south out of Ely to State route 318 continuing on 318 south and reconnect with US 93 continuing on US 93 to I-15 on into Las Vegas.
That's the route we take every year from Vegas to Birch Bay/Blaine WA.
Here's a link to the route we take:
Google Maps
Best of luck and safe travels...