download the vanilla behavior pack (for the latest beta)
modify the vex movement speed (minecraft:movement) to a insane number (eg: 100)
create a new world with the addon
spawn a vex
you should see the vex is the same speed as before(as if the vex ignored the addon). Both the vexes charge speed and the idle speed are both hard coded.
please note that this is only for movement speed and attack damage/health can still be modified.