Why not use http://www.viamichelin.fr to calculate your trip - you will get a choice of routes and timings (including the costs of any tolls). That way you'll have a reliable route planned with integral timings.

Hi There!
St Jean de Monts is not beside Vannes. Vannes is in Brittany & St Jean is in the Vendee. It's been a long time since I've made the journey, But I reckon Roscoff to Vannes would be about 2 hours and maybe another hour to St Jean. Maybe some-one who has travelled there more recently will be able to be more accurate but hope this helps.
[BTW Roscoff is a lovely little town and worth spending a few hours if not more]

Thanks Toasted Mallow ( love your thingy bear ? !) & Alsacienne. Yeah, I meant to type Vendee, I don't know why I typed vannes, done in another 2 websites don't even know where Vannes is. Anyway, my journey is 4 hours and 17 minutes to be precise!!!!!!!!!!!!!!!! Probably, 6 in our case, surely getting lost somwhere on the way!

Glad you got sorted. Have a great holiday in France. We had some wonderful times there when we caravanned.
BTW I think (s)he's a cat?! Just like me: Ready for the off at the drop of a hat!