The only "fake vegetation" I use is jute cut up and frayed, and placed on my rifle barrel/scope so that it looks like the long grass and hays you find in fields. I do this during the winter when most vegetation is dead/too short to be used effectively.
I would use jute and then maybe add a little twine in with it. I've seen a "fall" and "desert" ghillie that were pretty effective with colored jute strips and then the tan twine strings spread around. I wouldn't go full twine though; jute is cheap and works well.