My experience with the cheaper VEDs is that the cylinder walls are considerably thinner. This can result in more discomfort when the cylinder is sucked up against the pelvic bone. Some guys purchase penis sleeves to serve as cushions to reduce that discomfort. They can also reduce the hinge effect when used with constriction rings for sexual activity.
My personal experience has been that taking viagra, etc. beforehand helps allow blood to be pulled in by the pump. Viagra doesn't to shit for me by itself but it does very noticeably help with pumping.
I've also found that looking at nekid women pics seems to release the valves that make it difficult to pump up. Alternatively, if you can get your wife or girlfriend to help and play with your junk while and between pumps it can make a real difference.
None of those suggestions could work for you but it's worth a try. I'm post Radical Prostatectomy and have nerve damage and I find it difficult to pump up using a VED by itself. I have no natural erections of any kind or degree but it still seems I need something (visual or physical stimulation or pills) to tell the valves controlling the blood vessels that it's OK to let blood in. Even then it won't pump it in on it's own. Without supplementing the pump by one of those methods it takes too long to pump up an acceptable amount in a reasonable time without causing hematoma or edema issues. Using the methods outlined above I can achieve a nice hard erection with a VED.
Otherwise, thank goodness for injections.
Edit to add:
"Overall, there is convincing evidence that VEDs are successful in the treatment of ED following RP, especially when used in combination with PDE-5 inhibitors, which appear to work synergistically to overcome postoperative changes and aid penile rehabilitation."
The Use of Vacuum Erection Devices in Erectile Dysfunction After Radical Prostatectomy
by Hoyland, Vasdev, and Adshead
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3784970/