Well, it is not really necessary to call MPC or CX agent to get the guarantee V class seat. Any travel agent could do this too. If CX sell V class on the route/date you want, call your travel agent and tell him/her that you are CX gold member and offer him/her your MPC card number. The travel agent could send V class request to CX. Usually, CX would confirm the request in the same day.

However, please note that CX could NOT confirm any code share flight, even though the flight is operated by CX or KA themselves. For example, KA operate KA452 from Hong Kong to Kaohsiung and CX code share this flight as CX5452. In order to get the V guarantee seat, you could only book KA452, not CX5452 !