As astronaut is always to choose the food of his/her own choice . They have a tasting period of 15 days to eat and choose what foods he/she likes to eat in space. Of course there will be moderation by nutrition experts to ensure the astronaut is getting the recommended diet value of car/calories and so on.
Kalpana Chawla, a vegetarian stayed in space for close to 31 days. There are a few other people who stayed much longer than this.
P.S.: Not every astronaut is going to be vegetarian but if they are going to Mars they should become vegans. The Mars living room test (underground test) for 1 year included only vegan foods.