towns in Bretagne

Where are you coming from? How much time do you have? Rennes is a bigger city but it's the most centrally located for visiting the four towns you name and it's a nice city in itself with lots to see an do. It's also the best train connection to/from Paris and other places in France and it's also the most likely to have the best rental car availability. Since you're looking for a home base I'm assuming you want to see the other towns as day trips so it's hard to say how much time you should spend in each town. I've not been to Vitre so I can't speak to that, but for me Fougeres and Dinan were both satisfying as a single day visit - both well worth the visit but I didn't feel that a multi-day stay was necessary. On the other hand I felt that St. Malo was worth at least an overnight, if not two.

I agree St. Malo by the beach there is worth a couple nights to see the sunset and tide roll in at dusk.

Vitre would be a one-day town for most travelers, I think. I agree that Rennes is worth a visit itself.

You can check driving times on ViaMichelin.com. There have been positive comments on this forum about three major car-rental consolidators: AutoEurope, Kemwel and Gemut. Contacting them to check on your rental options would be a good idea.

We spent 6 days driving through Brittany and loved it. We picked up and returned our car in Paris but I might do that differently next time. Perhaps pick up in Caen and drop in Chartes.
We spent 4 nights at MSM at a wonderful BB that had a great view of the island that we would walk to at night after a day of sightseeing. We did Dinan, St Malo, Emerald Coast, Saint Suliac, the Rance River... as day trips.

We then drove the inland route visiting some parish closes to Auray where we spent three nights with day trips to Vannes, Carnac, Quimberon, Roch-en-fort-Terre. It was an easy drive and no tolls!

I like either St. Malo or Dinan for a base in the north, from there you can see St. Malo, Dinan, Cancale (great oysters and coast), Mt. St. Michel, and Ft. La Latte (nice medieval castle overlooking the sea.

I like Vannes for a base in the south. It's got a great medieval center and port area and is an easy drive to Carnac and the various prehistory locations. My favorite is Cairn Gavrinis, which is accessed by ferry tour from Larmor-Baden (about 8 km from Vannes). It's all in French but IMO by far the best tour in the area (and little known too).

I think I'd want to add something west. Brest didn't seem like where I wanted to go. If you want a larger city go with Quimper. If you want a small town, try Locronan.

There are also forests in the middle, if you like the Arthurian legend, Brittany has their own claim you can explore.

If you are there any time May to September, plan to visit a Pardon. It's a town festival that virtually every town holds sometime during those months, traditional costume, music, dance, and food. There's likely to be one in the area.

The best bet for renting a car is to train to a bigger town, probably Rennes, then drive from there. Afterward, assuming you are starting and finishing in Paris, you can return the car there and train back to Paris. Do something similar depending on what fits your itinerary. Once you get out of Paris, be aware that most agencies, even in bigger cities like Rouen, will close for lunch from something like 11-1 or 11:30 to 1:30. When you do an internet search, it won't show the place as an option. Search with pickup and drop off times around 10 am, then you can adjust once you find the places and their hours.

You could rent a car in St Malo or Rennes, and choose a base right in St Malo or around : depending on your wish to be along the coastline (St Malo, Cancale, St Briac, Dinard...) or more in the middle of the area that you are considering to discover as day trips (Dol de Bretagne, Combourg...).

We just spent 10 days in Bretagne in May. We stayed in a small town near Quimper. It was 10 KM from the ocean, with many small towns in the location. We stayed in a very small hotel (Hotel les Voyageurs) in Pleneour-Lanvein. It was very nice. The small towns in the area are all small fishing villages. This part of Bretagne (Finisterre) is very nice, very rural, very much farming. But at our hotel, we had our anniversary dinner - 31 E for the menu - just great. We thoroughly enjoyed this small, rural, agricultural part of France. We rented the car in Dinard, and then returned it in Chartres. It was Enterprise. They were very helpful, and dropped us at our residence in the city of Chartres.