I have chartered on Lake Champlain for the past 2 years. Last year I used Navtours
and I have a Beneteau 311 booked for the first week in August. from them again this year Actually I am setting up a blog on sailing charter vacations but I am still about a week from launch. Navtours is out of Mooney Bay which is just above Plattsburgh, NY. The only other Charter company on the lake that I could find is Winds of Ireland
out of Burlington, VT, but I don't have any experience with them.
Send me a private message and I will give you all sorts of info including contact names, boat owners, where to get guides, charts, etc. and a few great places to anchor.
BTW, my wife and I are fairly new to chartering and Lake Champlain is a low stress place to learn. No tides, usually good wind, lots of water, deep, and last year in August we had daytime temps in the 70's and evenings in the high 40's and low 50's. (That said however, the week before our charter, it was abnormally hot).
Hope this helps.