Have you forgotten that France is one hour ahead of the UK? I've just tried some random dates and times on the Speedferries websites. It showed, for example, that the 1040 sailing (BST) arrives at 1235 (CET+1). That's a crossing time of just 55 minutes.
The best way to plan your route is to use the ViaMichelin site:
http://www.viamichelin.com/
You've not said exactly where in the Vend�e region you'll be travelling to but, taking La Tranche-sur-Mer as a typical example, ViaMichelin quotes the distance from Boulogne as 661km (411 miles) and estimates the driving time as 6 hours 13 minutes. Total toll charges are quoted as �37.10.
If you live close to Dover, you might not need to include an overnight stay in your journey plans but, either way, you need to stop somewhere between Boulogne & your destination. The obvious choice has to be Rouen. (It's the capital of Normandy and well worth exploring):
http://www.rouentourisme.com/default.asp?ext=e n
If you do decide on an overnight stay, I'd strongly advise finding a local hotel in somewhere like Rouen - just use the 'Accommodation' link from that website above - but if you simply want to find a really cheap room for the night, directly on your route, check out what Formule 1 can offer:
http://www.hotelformule1.com/formule1/index.ht ml
Chris