Diagnosis
The locking mechanism is worn out. Either the plastic clips inside the brush head are broken, or the ridge on the handle's drive shaft that they snap onto is worn down.
How to Fix It
First, try a brand new, genuine Oral-B FlossAction brush head. Pop it on—you should hear and feel a definite click. If the new head stays on, your old brush head was just worn out. That's normal; they're meant to be replaced every few months. If a brand new head also pops off, then the problem is with the handle's drive shaft. Inspect that square post for cracks or excessive wear.
If That Didn't Work
Make sure you're pushing the brush head straight down onto the handle until it clicks. Don't twist it on. The Vitality uses a straight push-and-click attachment, not a twist.
Heads Up
Using non-Oral-B or generic brush heads can sometimes cause poor fit and popping off, as they might not have the exact right clip design.