If fruits and veggies are grown in selenium-rich soil, they will have adequate amt. I think, I used to take it all the time, it's supposed to help your heart. I'm not that familiar with it to comment with any confidence. It's basically more protein and less carbs in the form of pasta etc and lots of fruits and veggies. Diet did not help Linda McCartney.
Low fat is best, because some vegetarian recipes use a lot of cheese.
Remember breast cancer is multifactorial with a strong interplay between genes and environment. There is a lady named Rose Reisman who has written several cookbooks which are quite good and the sales benefit the Y-Me Organization. I have just bought her low fat vegetarian book. Her pasta one is fabulous. I am from Lima, Peru.