This is how I use my gravel vac -- First I fasten the end that the dirty water comes out of into my bucket with the bucket clip. The bucket is placed on the floor. The bucket must be lower than the tank for the suction to start. I submerge the vac end (the long, inflexible tube) into the tank and get all the air out. Lift the tube straight up and and about halfway out of the tank water (open end of vac pointing up) and let some water go down the flexible tube. When the water starts to move down the tube, submerge it quickly in the tank again, and turn the end of the tube toward the gravel. Suction will start, and you can begin vaccing (is that a word?!)
My gravel vac is a mini Top Fin brand. It's only 6 inches long in the tube end. As long as you put the bucket lower than the tank, you should be able to start suction without submerging the whole vac and tube in the tank. Maybe this method will not work with the tube vacs that are much longer. Although I have a 5 gallon tank now, I used this method when I used to have a betta in a bowl. It was easier than trying to submerge the whole tubing into a one gallon bowl!