American Vagrants ? Mmmm. . . .very little chance in April I suspect but you can always be surprised ! We've had Stilt Sand, a few White-rumped and a long-staying Long-billed Dowitcher (at Trevaly near Guerande and still around last week).
There was a Ring-billed Gull at Penestin and this area (from Pt. Du Halguen to La Mine d'Or) also holds several pairs of Dartford Warbler in patches of low scrub along the coastal path. It's a busy area in summer and the birds require some patience, often heard first before being seen..
The Foret du Gavre is superb - Honey Buzzard, woodpeckers (Green, Black, Great, Middle and Lesser Spotted), Tree Pipit, Nightjar, Dartford and Melodious Warbler, maybe Hawfinch and Golden Oriole. Worth a full day to explore the different areas / habitats within the forest itself.
Briere - superb again: a half-day at least for Whiskered Tern, Black-winged Stilt, Purple Heron, Savi's, Bluethroat.
Guerande - could be combined with Brier to make a full day or a day on it's own again. Saltpans with breeding waders, stilts, etc. Coast nearby could produce a good migrant in April, particularly at La Croisic.
Stuff can pass through quickly in April so if you fortunate with the weather you could do well . . . . . Good luck !
Sorry I can't really help with areas closer to Vannes but at least the above are within 100km.
I've seen Hoedic and Houat from a boat only though I did spend a few hours on Belle Isle (which is BIG with plenty of cover for migrants). Nice place but I don't remember seeing special there that I wouldn't expect on the mainland.
Eugene
PS: let us all know how you get on.