I don't have a VP9 but I have a P30, which has the same grip system where you can modify the backstrap and grip sideshells to fit different sized hands better. I think this is a terrific system and I think I shoot the P30 better since I modified the grip to fit my large hands. The VP9 and P30 also share magazines, which at least partly explains why replacement mags are so hard to find, and so expensive!
I also have had a Gen3 Glock 17 for the past 6 years and I have put thousands of rounds through it, so in my mind that gives it an unfair advantage over the newer HK. I'm a big HK fan and I think HK build quality is slightly better than Glock build quality, but Glock is a very close 2nd. I think the VP9 will meet all of the expectations that you have for HK reliability and durability. In the long run it's probably every bit as good as the Glock 17, but I'm not sure "yet" that it's better.