If your focus is to be on the Desolation Sound area you'd be best off to charter from Comox for proximity to the area. You can easily spend a week sailing to and from Desolation from Vancouver. From Comox it's an easy one day trip into Desolation, leaving you much more time for exploring than having left the Nanaimo/Vancouver area. 10 days would be a pretty good stretch using Comox as a base. You'd have plenty of time to explore the area.
A week in the Gulf Islands would be a nice taste, and if you're OK with anchoring you can avoid some of the crowds by carefully choosing your overnight spots. The only downside to the Gulf Islands, really, is that the water rarely warms up and you need to be pretty dedicated to do much swimming in the sea. That's another Desolation attraction, btw, many warm lakes within easy walking distance from good anchorages. (few of them are true 'hikes')
All the same, there's plenty to see and take in between Vancouver and Desolation too, including parts of Howe Sound, Jervis Inlet, the islands between Texada and Lasqueti mid strait - and the Hornby/Denman islands as well; Tribune bay is a fair weather jewel.
August is generally a good time for the inside waters, but foggy beyond Desolation and on the outer coasts. I have to say that in the past few years we've rarely felt 'crowded' but we do avoid some of the 'traps' like Prideaux Haven and other popular hideouts.
If you haven't seen it already, have a look a this post, laying out our itinerary last summer..
Our 2010 Cruise (pics)
If you search around you'll find other similar reports from myself and others in this area. Also feel free to send me a PM if you'd like more info... You'll be glad you took the time to explore this amazing cruising grounds.