Oh yes, the wonderful St. Nazaire bridge!
I hate those suspension bridges and I'm so glad I'm a passenger.Only thing I can say to you is travel on a Sunday morning when it's nice and quiet and there's little traffic.At least you get across quickly.
otherwise,just focus on the car ahead of you and follow it without looking down or left/right.It is horrible.
My husband says you can avoid the bridge by going on the motorway to Nantes and avoiding St nazaire. Take the N165 to nantes and head towards airport and go onto D751 and will bring you towards Pornic and other areas in vendee.You're adding perhaps 45 mins to your trip.

Sins

Don't like the sound of that bridge! Would rather take 45 mins longer than go over that

If it is the St Nazaire bridge you mean, then you could miss it by carrying on to Nantes. However, you would then find yourself on the Pont de Chevire in Nantes - another very long, high & windy bridge. The views are superb and the bridge itself is an engineering masterpiece. However, I can appreciate that some may find it as bad as the suspension bridge in St Nazaire.

If you really want to avoid it, though, you could go clockwise (instead of the usual signposted route which is anti-clockwise) around the Nantes ring road, i.e. turn left at the end of the N137 from Rennes.

See http://www.viamichelin.com for good maps.

Also, Angela Bird's excellent website has very good driving directions to the Vendee:

http://www.angelabird.plus.com/routes.htm

- Tony -