Studying the rules, I think this needs to be a combat maneuver--a version of the charge + overrun.
1. Instead of allowing the defender to chose to avoid the overrun, there will be a check, if he wants it: Most likely, this check will be defender's DEX or Reflex vs. vaulter's Jump. But, the defender will have a choice on whether he wants to try to avoid or not. He can stand his ground and do nothing if he wants.
2. The defender will get an Attack of Opportunity on the pole vaulter as the jumper enters the defender's square.
3. The pole vault attempt will be based on Jump, with maybe a modifier (maybe not). The attempt itself needs a DC, because the vaulter could have a mishap, like having his spear slip in the mud, not bearing his weight, or some such calamity.
4. The rest of the maneuver will be run according to the overrun rules to where the defender or the defender + attacker may be knocked prone.
5. The idea of the attack is to have a spearman run at an enemy, then use his spear to pole vault into his target, knocking him down, ending the action standing above the enemy with the enemy prone.
6. In my campaign, this is going to be a specialized attack known by a certain Cimmerian tribe--the Grath. It will be known as the "Grath Attack".
EDIT: Maybe the Grath should also use some sort of special construction spear? Something that makes it flexible? Are we talking a long war spear or a shorter hunting spear? The length of the spear will determine the distance.
All of these questions need to be answered. I lean towards the war spear because, first, this isn't a maneuver you'd use while hunting, and second, the war spear is longer and not meant to be thrown. Therefore, it's construction can be a bit different, maybe made from a strong cane that grows around the Diamondrun River in the Grath territory?
Food for thought.
2ND EDIT: If this is a specialized Grath manevuer...
I picture a Grath, teeth filed down to points, kilt made of reptillian skin, with scars on his arms and the clover tatoo on his forehead, running at you, screaming a war cry from the bottom of his chest, as his spear digs into the ground...up he goes! His arm muscles straining. His mouth open, screaming the entire time. And, two booted feet sailing directly at you.
What about working and Intimidate throw into the Grath Attack so that the defender may be shaken that first turn?