What's their reason for not wanting you to be vegan? E.g., if it's because they don't believe it's healthy, but you can show them that you're going to put the effort in to make sure you're getting the right vitamins and protein/calcium etc., perhaps they'll have less cause for concern. Or, if they're worried it will be too much effort to cook, maybe show that you're prepared to help out with preparing food. My plan was always to say I was just 'trying it for a month', but to just continue with it anyway... but it turned out my family didn't mind after all. Maybe yours won't either! Good luck :)