Brittany & Normandy in 2 weeks

I've visited some of those cities. RS books don't cover everything worth seeing; the Lonely Planet book is what we used, and for anything over a week you'll want the detail in the Michelin Green guides (one each for Normandy & Brittany). On the Fodors site there is a regular poster named Stuart that will send you his sample itinerary on request; we used it and found it helpful.

Vannes is a nice city but be aware it isn't on the coast exactly; there is a channel from its harbor to the bay that is too far to walk. If I was doing it again I'd pick a town actually on the water, but that's just me. Also you aren't visiting Rennes and that was one of my favorite cities in the region.

RS is right about MSM; arrive late afternoon when the crowds are leaving, stay the nite, visit again early morning and then leave. We stayed just off the island. There are frequent shuttles across the bridge, and you get the great view both day and night. Others want to stay on the island, its a personal choice.

You don't have Bayeaux on your list. Great place to see the D-day sites if you are interested, and also the fabulous tapestry.

Depending on the road you choose between Vannes and Camembert, it may be between 3h45min (partially on "highway") and 4h30 min. The most direct route, which is also smaller and slower, takes you through Fougères which is a beautiful town and can be a very pleasant break for 2 hours.
Your itinerary sounds good. Since you want to see both eastern and western parts of Brittany, 2 nights in every of the mentioned places is good, not to spend too much time driving, whereas there is much too see in a small perimeter around each place. On the way from Dinan to Roscoff, you could enjoy some time along the pink granite coast. You may find some ideas to complete stages in your itinerary on http://visite.bretagne.free.fr/index.php/en/itineraries.html

I would try to make it more of a loop. Start south and work your way back up toward Normandie. The drive from Mt. St. Michel to Bayeux is only 1 1/2 hours to give you an idea. You can use via michelin (viamichelin.com) trip planner to estimate the drive time and tolls.

My favorite places in Brittany (starting from Vannes and going around clockwise) were Vannes, Carnac, Cairn Gavrinis, Auray, Pont Aven, Locronan, Lampaul-Guimiliau/Guimiliau/St. Tegonnec (for their unique parish close design), Cap Frehel (if you like seabirds), Ft. La Latte (great old castle), Dinan, St. Malo and Cancale (truly great oysters). I ended up avoiding some of the bigger places - Concarneau, Quimper, Brest - because they seemed too big and industrial for my taste.

Direct trains between Rouen and Paris are frequent.

"Depending on the road you choose between Vannes and Camembert, it may be between 3h45min (partially on "highway") and 4h30 min."

Just keep in mind when planning your journey that those times are actual 'drive' times under ideal conditions. When you add rest stops, maybe a lunch stop, possible traffic, road conditions, weather, you need to add a couple of hours at least to those times. Having driven in both Normandy and Brittany (and other regions of France and elsewhere) I have found that recommended drive times between two towns were less accurate the longer the distance between the towns.

Even signposted the road to Château Gaillard in Les Andelys is not so easy to find. You have to exit the D316 and take the D1 opposite the church. A few km further uphill you have to look carefully for the –signposted- narrow road leading to the parking of the castle.

Worth a detour there is 25km more north Lyons-la-Forêt, nice countryside to explore btw.

We have travelled by car three times in Normandy and Brittany. I am always amazed Rick has so little coverage of Brittany. For planning we used Frommers 25 Great Drives in France. It has the country broken into 3-5 day trips on small roads. At least two of them cover the areas you are travelling. our last trip was in September 2016. We had no reservations, rented the car in Rennes, used our guidebook and the Michelin map and had a blast. Usually we would make the reservation for tomorrow night today, but some days just found a place in a town we liked near the end of the day.

After we used the guidebook, we web searched for things we might want to see in each area an made a list before we left the US. Then just drove into an area, say like Quimper or the Gold Coast, and hit the places as we went. Each night at the hotel we always got some gem of a thing to do the next day. so that flexibility really helped. Have fun! Enjoy the seafood!