First things first - 300 calories 3 times a week is only enough to burn quarter of a pound in a week, so you wouldn't expect to see a difference on the scales. Even 400 calories 4 days a week is less than half a pound. Are you exercising just to increase your weight loss? It's a bad idea to use this reasoning when you're on a VLCD because you don't have enough fuel to do the kind of workouts that burn so many calories they make an impact on your weekly weigh in.
You need to drink between one and two extra litres of water on days when you exercise in order to adequately rehydrate. People tend to grossly underestimate how much extra water they need. If you don't properly rehydrate you will retain water and slow down your fat metabolism (very simply, your kidneys need water, and if they don't have enough then the liver has to help them out which means it can't do its own job efficiently).
I'm guessing the reason for a slower loss this week is that you're not drinking enough water, but even if you drink plenty of water, you're not going to see a huge increase in your losses.
Exercise is a great thing and if you are keen to be active I'd recommend sticking with it and moving up to the 1000 or 1200 calorie programme as Icemoose suggests. This will give you enough fuel to work hard and burn more fat. It might sound crazy, but you'd be able to work harder at the gym, eat more, and still see similar results.
I can give you as much science as you want
but I think this pretty much covers it.