It is a bit like asking how long a piece of string is but there are some things that can be mentioned in my view.
If you are going to reduce calories you do not have to restrict nutrition which is all important.
The most nutritionally dense foods per calorie are essentially green leafy vegetables on the whole. Well leafy vegetables that are not green like rainbow chard are in there as well
Salads and green smoothies are right up there as fillers in terms of dietary intake in my view!
If you like fruit to be a large part then I would recommend that you have a meal of this on its own rather than at the same time as your leafy greens.
Personally I would recommend the vegan version of things but if you are committed to vegetarian then it is still a step in the right direction. A few more steps would be great!