Here's a couple of differences:
Vegetable cleaver generally has a longer blade giving you more slicing action.
Vegetable cleaver is larger allowing you to scoop more ingredients at once.
Vegetable cleavers also tend to have square tips at the front, better for slicing into stuff, whereas the nakiri's tend to be rounded, better for rocking on the board.
You can find a veggie cleaver of medium weight capable of doing small bones easier than finding a nakiri that can.
And this is weight, grind, and steel differences aside.