You can trim it back some yes, but if you also train the vines to grow horizontal they should put on new branches who will try to grow up- Thus giving yourself a fuller plant.

Be sure not to watch it every day tho as then it will appear to be doing nothing.

I'd say some varieties aren't terribly bushy by nature without some help and need lots of pruning and perhaps the horizontal treatment mentioned to encourage new branches. It's only larger specimens with big "rootruns" that can get really bush usually and then those can cover over a huge area. Give it time for lots of root growth and it will get bushy if it's in open ground where the roots can spread.

what kind is it?
I have an Edulis, covering a 30 foot fence and growing up in a tree besides.
It"ll die back in the winter, but comes back even larger.
This is supposed to be a zone 10 vine, I"m in Gainesville, fl.

My advice? Leave it alone. Stop watching it & let it grow. 'Cource I'd cut any vines who are growing out of the defined area, but other than that they grow better when left alone I've found.