This vine can easily grow 20′ in a season so there is much to be cut back. It had also gotten quite top heavy last year.
As with all vines, cut out all the dead vines, then all the thin vines, and just keep going until you only have a few strong vines left.
This one is especially a pain to prune because the tendrils it sends out tangle it up.
Flowers are only on the new growth so be brave cutting the passion vine back.
Passion flower vine