Was in Vannes only last week and it is lovely. Not sure I’d recommend it as being “beachy”. We tooled around the Morbihan by boat, and a lot of the beaches are pretty small and/or have Oyster beds nearby. That said, the Ile Arz was very nice.
I can also recommend St Malo. We had a couple of days on a campsite just outside. The swimming was nice and the town, despite being heavily bombed in ’44, is worth a visit – you wouldn’t starve that’s for certain.
Further along the North Coast the region around Perros Guirrec (sp?) – the pink granite coast – is beautiful.
Finally, at the very limit of your range, we had a couple of holidays in Benodet when the kids were younger, and the area around there is pretty nice.
LR