Dragoon, Full attack or Vital Strike?

[Viladin]

Yeah thats what I figured, so the Vital Strike tree would be needed to keep up with other classes. Does the vital strike bonus double from 'Leaping Charge'?

Nope. Can't use Vital strike with a charge.

Can Vital Strike be used with Spring Attack? Can Vital Strike be used on a charge?

No. Vital Strike can only be used as part of an attack action, which is a specific kind of standard action. Spring Attack is a special kind of full-round action that includes the ability to make one melee attack, not one attack action. Charging uses similar language and can also not be used in combination with Vital Strike.