The grapevine is usually used to stretch you out and remove your base. It is also used to get enough space so you can remove them, and get open and butterfly guard going. Some sneaky bastards use it to set up leg locks, specifically a really nasty side kneelock.
If she is sweeping you using this move she must be VERY strong. More than likely she is taking your base away, then getting a different position to sweep you. You are still stuck on what just happened, rather than what is happening NOW. Honestly, grapevines are good for control but not that good for changing position.
Jlau already said bring your feet under your butt. That makes the grapevine tough to do. Another way to avoid this is to go knee up in the middle.
Since you sound like a relative beginner let me describe what you are doing. You are using superior size/strength to muscle your way on top. She is using skill to put you back in guard. You are concentrating so hard on 'keeping her down' instead of learning to pass the guard. In the process of keeping her down, you are applying a LOT of pressure, and she is using that pressure to leverage you off. What you need to realize, which is easier to say than do, is stop trying to keep someone down and try to pass into a better position which will actually put them on defense. Think about it, what exactly are you doing there in the guard? You are probably just watching the bottom person work. Get out of that mentality. Because the problem you are describing, and the way you described it, you just watched the other person work you.
Grind your manhood into her fiercely...
Thanks guys. I realised after I posted that I must have been coming up off my heels way too much. I did try to take advantage of the knee being low but I didn't know how to safely untangle a leg. Yup I am a beginner and my guard passing sucks. Anyway i'll work on those suggestions, cheers.
(I didn't ask my instructor because class was finishing up, I was tired, and I wanted to go home)