This. If you are sitting VDO you are not required to answer your phone or accept a trip when they call- they just offer it to you & you have the option to pass or play. If you are on a trip that touches a VDO (ex: working a 4 day Mon-Thu w/ VDO on Fri), they can call you at any point during that trip & either assign you work on Fri, or just roll you into a 5 day working block so you no longer get to go home on Thu- all without your consent. Basically, waiting until you are off duty to convert to VDO preserves the element of choice for you.