I'm a veggie (vegan at home, but I don't have the time to be wholly vegan here, unfortunately). These are my recommendations so far, for antananarivo. Has anyone got any other specific recommendations to share, for anywhere in Madagascar?
Eating out: The chinese restaurants are lovely, and will do you a vegan meal, with tofu (fromage de soya), rice noodles (something like blanche...). I had a nice, but overpriced (it was the price of a cheap chinese in the UK, 25,000 Ar or about £8, for starter, water, jasmine tea & main course) meal at Jasmin (on google maps, called Jasmine, http://maps.google.mg/?ll=-18.910161,47.526568&spn=0.001553,0.00246&z=19&vpsrc=6), behind the BNI-CL bank.
Eating in / picnic: You can buy nice veggie samosas from the supermarkets, which are called "Shoprite" and can be found by searching google maps. You could also obviously buy bread and cheese, olives etc. but no humous. they also sell marmite, at uk prices! http://maps.google.mg/maps?q=shoprite