I was in the Morbihan last weekend and early this week, albeit on a sailing yacht, on a 3 week cruise from Poole to Southern Brittany and back to Poole again. An absolutely stunning place with loads of interesting anchorages to explore, a variety of islands to visit, etc. Vannes also a wonderful city, loads to see and do, plenty of shops, restaurants and cafes.
Sadly because of time constraints and a requirement to stop in Vannes to pick up Mum, we didn't get to visit any of the islands, but sailed past some stunning bays on our way up the harbour.
Not sure what launching facilities there are in Vannes - there was a clearly very old slipway right beside the Capitanerie, but that seemed pretty much closed off now. There is a boat yard with a crane just down the Vannes canal, which could presumably drop the RIB in for you (Chantier Analyse des Carenes de Vannes). Vannes itself does get very busy with serious rafting going on, and given they are probably more used to visiting yachts might be worth giving them a call/email to ensure they can find a space for a visiting RIB - the girls in the Capitanerie all speak very good English if you don't fancy trying in French! Only downside is small craft seem to get put down the far end beyond the swinging marina foot bridge, so are stuck there when the marina foot bridge is closed. They are still digging the new tunnel right beside the main Kerino swinging road bridge which makes it quite a tight and bendy route in - absolutely fine in a RIB, but best avoided at the bridge opening times when yachts are queueing up to get through in both directions.
Given the number of French who visit this area for holiday, there's bound to be various sailing clubs, campsites, etc. with slipways in the Morbihan - probably worth hunting around the web, or asking the Vannes Capitanerie if they have any info.