320 > VP9. For competition, you would have to deal with the HK paddles for quick reloads. That would probably put you in a disadvantage. Also, the VP9 feels less robust and lower quality than other HKs. The 320 does everything well enough.
In competition, someone who trains hard with a VP9 will absolutely destroy someone shooting a P320 who doesn't train. Substitute any two reliable guns (even open guns vs single stack) in the previous sentence, and it holds true.