Okay, so obviously you know the consequences of crash dieting. So I'm not going to tell you how detrimental it is to your health. I don't know what your weight/height is, so of course I can't tell you whether you indeed need to lose weight or not, but I just want you to be careful. I know the exercise and eat healthy thing sounds cliched, but it really is your only option, especially at this stage in your life. You need to take care of your body. It is your power house, it is what will get you through the next two years. So, having said all this, I would recommend eating frequently throughout the day. Sounds odd, I know, but if you manage to eat 5-6 times throughout the day, you will keep your metabolism going, and will remain concentrated and energised. So rather than cutting down, try to focus on the types of food you are consuming. For instance, at breakfast, aim to have wholegrains. Try something like 2 weetbix/2 pieces of toast with some fruit salad and a few spoons of yoghurt. Have a mid morning snack, something like a muesli bar or some vita wheats and low fat cheese. For lunch, have a sandwich with a protein source, and perhaps some fruit/ milk etc . Afternoon snack, try something like a low fat yoghurt, to help boost your energy. Other snacks may be hommus and vegie sticks/rice crackers or nuts and dried fruit etc.
Sorry for the rant, I hope this makes some sense. I just know where you are coming from, and I dont want you to make the same mistakes I did. I went too far and have paid the price. Remember its good to have chocolate etc. Just try to keep balanced. Make time to go for a walk, do some study then let of some steam by going for a jog around the block. It helps.
And yes, maybe it may be better to wait until after vce. It is one less thing to have to concentrate on. And changing ones intake can definitely affect concentration in general