Vancouver Island

Either Tsawasswn or Horseshoe bay terminals will get you there but will be very expensive. It would actually be cheaper to go from Port Angeles even with the extra distance involved.

I would use Tsawwassen in that case.

Back & forth from Vancouver. Looking at more than 10 days on VI.

Many options.
If you are visitng Washington's Olympic Pennisula, I would suggest Black Ball Ferry out of Port Angeles to get to Victoria. Then come back to mainland fro Nanaimo to Vancouver.
Another thought would be to leave rig on mainland. There is very good transportation on Vancouver Island. Do a walk on and spend the night or two in hotel.

From where?