20 shots is way too low for your Vet. I have a .25 compact and I’m getting 30 shots with 25.4 grain with speeds in the 880’s which is right round 44 foot pounds of energy. My air tube is the same size as yours, so something isn’t right. I would start by backing off the hammer spring which is the brass adjustment knob you are talking about. Make sure you back out the allen head retaining bolt on the left side, this keeps the hammer spring adjuster from being easily moved. Insert the two pins that should have come with your rifle and move about 1/8 of a turn counter clockwise. Then shoot it over a chronograph and see where it’s at. I would suggest around 880 FPS. I would do small adjustments until you get it where you want it. Tip - If you need to increase the velocity by turning clockwise, the rifle will be hard to cock if you went too far. Definitely back it off some if this happens. See my pic below for reference on the instructions above.