Motion in Vi is based on treating the home row of the keyboard as arrow keys. H, J, K, and L are left, down, up, and right. They aren't mnemonics, and so scrambling them around (Dvorak) is just doubly confusing.
Can confirm, I use Dvorak and vim without issues. I find myself not using hjkl very often though, since I can usually use another movement to get where I want to go, so it's really not an issue.