There was a diet head-to-head challenge on TV once, a series with Dr Christian Jenssen or whatever his name is.
One week they had a vegan lady against a GP lady. They liked to rev up the competition a bit so they had the vegan lady wanting to spend the prize money on some kind of organic toilet and the GP lady (who they made out to be a real meat lover) on some racing car day (but had shots of the vegan going "ooh, leather seats, eww, poor animal) etc etc.
Anyway, they were both the same weight, quite large, and both did different diets and quite a lot of exercise, but of their choosing.
I don't know how many calories the vegan lady was supposed to be eating, but she was doing daft things - like she'd go for a walk with her family to the top of some hill and they'd get to the top and she'd say "who wants a jacket potato?" and she'd pull some full sized jacket potatoes out of her rucksack and they'd sit there eating them...for a snack! (or so it seemed, who knows with editing, perhaps it was their lunch!)
But anyway, she seemed to be eating a lot of carbohydrates and not much else. I'm not anti carb necessarily but one thing is true - you either burn them or you store them. There is nothing else your body can do with them other than energy (and use the nutrients that come with them of course). So you burn, or you store as glycogen and fat, simple as that. Protein has many other uses for your body, apart from keeping blood sugar balanced by slowing down the carb hit of whatever you have with it, its muscle repair, tissue repair, all sorts of other functions (sorry if you know this and I'm preaching to the converted)
At the end they weighed them and they didn't just weigh their bodies, they were monitoring their body fat levels.
The GP won by quite a lot because she lost more fat - the Vegan however was losing small amounts of fat and a fairly alarming amount of lean body mass. The doctor warned her about halfway through to up her protein intake to stop this happening, but she was quite stubborn, I suspect she was sick of hearing about protein in general, being a vegan (and you can get enough protein for your body from just plant foods, as we don't "need" as much as some people think, just for day to day stuff). But add exercise to the equasion and she clearly wasn't getting enough. She also seemed to be hungrier.
So the protein thing is pretty important. It's a very useful tool for me when I'm watching what I eat. Must be hard without meat or eggs but there are things out there! Good luck...