I used to use just a piece of hose but that most times will vac up the gravel into the bucket of discarded water.
A gravel vac simply has a larger entrance into the piece of hose (about an inch and a half in diameter) for the gravel and when the gravel is lifted it simply sifts out the lighter derby particles and allows the heavier gravel to return back to the tank.
Simple concept but there is many different varieties.
I think the simplest form is all anyone needs.
Tell me about it , i just went out and got a air pump and a gravel cleaner with an attached sack that puts teh water back into the tank only to find out it doesn't work very well, does clean the tank but not on teh level with a standand gravel vac that sphinon teh water also. Silly advice that a friend gave me..wasted a lot of money..