Check out this article with tips from a “short” Division I volleyball player with a 28-inch vertical. (I’d trust her… 28 inches is legit)

Breaking a Sweat: How to soar in volleyball

If you want to play volleyball at the elite level, you better start doing abdominal crunches now.

When it comes to driving the ball over the net, a strong core, it turns out, is as important as strong legs and shoulders.
“I played a couple of sand tournaments a couple of weeks ago and that was the first thing that was sore, my abs,” says Lauren Budde, middle hitter with the Washington University Bears.

As she describes it, players do what amounts to a mid-air abdominal crunch — picture the body in a V shape — that places their hands over the net and their feet beneath it when blocking and killing the ball. This allows them to transfer energy from the ground up through their body and into their arms and hands. They use the core muscles again to rotate off the net and get positioned for the next play when they land.
According to Dale Huff, owner of Athletic Republic Sports Performance in Frontenac, vertical jump is the big buzz phrase in volleyball because everyone wants to increase it.

He estimates that highly competitive male players can jump 40 inches off the ground.

Read more about vertical improving tips here.