We are considering this tour and wondering which tour date to pick. We are looking at end of August through middle of September. Why would you choose August start as opposed to September? Weather? Number of tourists? Other reasons?
I would not choose August instead of September. I would choose September instead of August. August is usually hot and also almost all French have their vacation in August.
I have done this tour in mid June (2015) as well as the first part of September( 2009). The hotel in Vaison does not have AC and it was unbearable both times, but more so in June this year. I would go as late in September as possible. The grape harvest should be done by then and hopefully the weather is starting to calm down by then.
I'd choose as late in September as you can possibly do.