Would eating pastries ruin my diet?

It really depends on the pastry. There are some vegan pastries that are gluten free and have less carbs so it won't really hurt your diet as much as say a doughnut. Either way the best way to be sure you stick to your guidelines is to count the calories of what you plan on eating and see if the pastry will interfere. By the way, if your interested in the vegan pastries they're usually easy to find in organic markets.