The 24-year-old singer's band mates Rochelle Humes, Mollie King, Una Foden and Frankie Sandford, told her to calm down and stop going out as much.
Vanessa told the UK's HELLO magazine: "I'd been partying way too much, not taking care of myself and I felt rubbish. We're all very honest with one another and we will sit each other down and say, 'This is getting out of control'.
"If I hadn't had them, it would have been so easy to have gone off the rails and kept up the partying until it became a problem. You need people around you to say, 'Right, this is too much now,' and fortunately, I have that."
The brunette beauty said she started eating more healthily, reining in the nights out and working with a personal trainer three times a week. Vanessa has now dropped a dress size in five months.
She added: "I have a trainer who gets me running, doing squats, lifting weights, and sometimes I feel like I can't breathe, it's so hard.
"Afterwards, though, I feel amazing and it's great when you start seeing the results. I'm never going to have a small bum, but it's toned now and that makes me happy."