You basically have three choices.
Celebrity, HAL, Princess all do LA, San Diego or San Francisco return with about 4 days in Hawaii.
Carnival and Royal Caribbean both do Hawaii - Vancouver (or Reverse) with about 7 days in Hawaii and 5 Sea Days.
NCL has Pride of America based in Hawaii and does the 7 day around the islands...no sea days.
Question is, do you want to do this for the cruise, or to see Hawaii? you'll get very little Hawaii with the first option. I've done NCL and loved the cruise, the ship was ok but for us Hawaii was the main attraction.
I'd never do the Mainland return, to me it's wasting two weeks vacation with only 4 days in Hawaii.
I would like to do the one way to or from Vancouver, 5 would be an ok amount of sea days and you get longer in Hawaii than the mainland return. This option also allows you to spend more time in Hawaii either before or after the cruise.