Chez Yoshi is in Chexbres which is about 10km. from Lausanne. It has very good sushi and has got very good reviews on local (i.e. French-speaking) websites. It is small and reservations are essential on the weekend. Expect to pay about CHF120 -150 for two people to include drinks and lots of different sushi and sashimi so it is not inexpensive.
The address is Rue du Cottard 2, 1071 Chexbres, Phone is 021 946 12 03