Many of today's vaulters do 1-4 crucial mistakes at the plant and take off that cause your problem. It sounds as if you have at least one of those problems.
Problem 1. If you lock out your bottom arm or press out with your bottom arm in a way that will block your shoulders from their forward momentum. This transfers energy into the hips. Now your hips will get out in front of your shoulders too soon. This also causes lower back pain.
Problem 2. Jumping off the ground in a way that it throws the hips out in front of your shoulders is not mechanically efficient. Even if you don't press with your bottom arm, it will cause similar problems as Problem 1.
Problem 3. Extending or kicking your lead foot forward in the take off or drive of the vault. This extends the body past 45 degrees when the body is fully extended in the vault. That is to say...When your body makes a straight line with your top arm, head, shoulders, body and straight trail leg, it should be at a 45 degree angle to the runway. That line should be pointed at the box. The trail leg foot should be tapping the box. The box is the bottom of the swing. The top hand, head or lead leg should never be forced down toward the box. ("Rowing" or pressing the top hand forward past the vertical plane of the shoulders is forcing the top hand down toward the box, but that's another story addressed in the Advantage Athletics web site.)
Mistake 4. Swinging the trail leg through.
See
http://www.advantageathletics.com/polev ... rough.html
You can see this position in many of the top high school vaulters. That does not mean it is the most mechanically efficient thing to do.
See:
http://www.advantageathletics.com/polev ... bubka.html
http://www.advantageathletics.com/ballentine.html
For a better take off and first part of the swing see Tim Mack at
http://www.advantageathletics.com/polevault/mack.html
For awesome see Kristna Molnar at
http://www.advantageathletics.com/polevault/molnar.html