Pruning passion flower vine

Does anyone know how far back I can prune a passion flower with out losing it. A shell vine got mixed in and has just about taken over...and I hate it so I thought if I could prune everything back I could than pull the darn shell vine out all together and let the passion vine grow back.....
I hate that shell vine so much..I purchased it as a cork screw vine bought 6 they were all shell vines..Ane here in Fl. they are wild growers they take over every thing and trail the ground(Have I said I hate it). If you know about the prunning let me know....And anyone that might want some shell vine I have about 2 foot ball fields of it...no kidding

It's usually safe to prune back a third without hurting the vine.

Cinnabor,

I've cut my passion vine to the ground, they all came back. They're taking over my fence, trees, etc. I even dug up a root ball that was larger than a basketball, it took immediately. I'm in 9b (around Orlando) and I have had no problems with them.