It depends on tickets.
For ex-JPN,
V classes for any HND-HKG-SIN-HKG-HND are sold.
Fro ex-HKG, if V class tickets aren't sold for patricular flights/itiernary, you can not use seat guarantee for V classes.
Another example.
For ex-JPN.
V classes for HND-HKG-HND aren't sold, so I can not use guarantee seat for this ticket.