Accelerate through the box. The body's center of gravity should be rising before the pole hits the back of the box. If right handed, the right hand should be pushing as high as possible, to allow for a proper swing around the right shoulder. Keep the left ankle as far away from the center of gravity as possible. If the plant and swing are properly executed, the a good top becomes automatic.
Speed is where it's at.
Weights are fine, but great vaults come from the speed and jumping ability, not big chests and arms. Build an endurance base, but work on a speed and agility base mainly. Pop-ups and plant drills. So try to keep the grip as narrow as possible. Then add 1'- 2' and work your way up as your skills permit.