Short answer - flexibility isn't the key factor in vertical jump/speed.
Long answer - Read this article. What matters is that you have the necessary flexibility to execute the given movements, and that specific muscles aren't overly tight or loose. Ex: The hip flexors (quads) are tight in a lot of people. If you can't pull your heel to your glute without abducting your leg out to the side, your quads are overly tight. Generally that will be more of a problem that affects sprinting speed than VJ height, though.