I would go north on land and use BC Ferries. Easy on-off, and better ride and more frequent crossings, so you can avoid peak hours easier than Black Ball. Washington to Vancouver can be rough as its cross gulf, so winds and tide can make it rough. Also you have to deal with customs at the dock both ways with Black Ball vs the land crossings, need an hour prior to sailing to be at the dock.
Also black ball is downtown Victoria.
We stayed in Nainimo and drove to see sights. Only about an hour scenic drive to Victoria. You should go to Duncan and see the totems.
Ive done both ferries, and overall BC were better and easier. Black ball is an older boat vs a newer fleet with BC Ferries. Rates may be better with BC.
Can do VI on multiple routes from Vancouver BC depending on your stay point.
You can restock on VI, similar prices as Seattle and most us cities. No need to take a lot through customs on either ferry.