Yes, I think when the unit, especially the base, is "sealed" with water, the suction will be better. As for standing up; I always sit down and hold the unit in an angle, making sure my cock is covered with water, not with air. (If there's any air in the unit.)
Vaseline might not be necessary for now, it's better to try to get a good seal without it. As for pumping, I usually do one or two almost full pumps and then gradually pump more. When you pump more the pressure grows. So as I said, don't pump too much too fast. You should get a nice good seal, feel that the unit is not going to come off easily. That usually takes 1 -3 pumps in the beginning. Then relax for a while and pump again. Keep an eye on your dick.
Also, for good seal your pubes should be relatively short. So if you have a jungly down under, try to trim it a bit. I noticed when my pubes were a bit longer and would come to the area where the [words=https://officialhydromaxpump.com/?uid=6&oid=2&affid=98 ]Bathmate[/words] seals, that the unit wouldn't stick on so well.
Have fun pumping, don't give up!