Jonathan, the deadrise of the hull is what determines how deep the vee is. For an explanation of this, have a look at this thread.
As I understand it, the deeper the vee, the better the hull will cut through
the waves as distinct from banging up and down on top of them. A bit of a rough and ready explanation but I'm sure someone will come along in a minute and explain it in more scientific terms!
Edit: I believe a deep vee is better for rougher water, whereas a shallower/flatter vee will generally make for a faster hull in good conditions. As with all things boaty, the perfect hull is a compromise!