Desolation Sound is at least 2 long days from Vancouver. That means 4 days just getting there and back.
August is probably the busiest month and one of the least windy.
A week in the Gulf Islands will give you a flavour of the area. Try to get as far as Ganges. Using Nanaimo as a starting point will require that you go through Dodd Narrows, which can only be done twice a day (during daylight). You might also consider Sidney as a charter base. There is an airport and a ferry terminal there.
Chartering out Vancouver means a day crossing Georgia Strait each way.
Tides and currents are big issues in the area. There is a 13 foot tidal range and currents in Dodd can run 8 knots.
There are great anchorages, good marinas and some of the best topography on the planet.
Three days is not really sufficient and you may not find a charter company that has rental periods that short.
There are several of us for whom this is our stomping ground, feel free to ask for any advise.
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As I sail, I praise God, and care not. (Luke Foxe)