If I understand your question correctly, no you cannot do this. As mentioned before, certain flights will not have V class (or even M or K, etc.) available. This can be due to a few reasons:

1.) V class (or another similar cheap sub fareclass) was available at one time, but they had time-specific purchasing restrictions and hence it's no longer valid (say, ticket must be purchased before November 30, 2009....in this case, you would not be able to force yourself into V class if you were trying to buy a flight for next week)

2.) V class (or another similar cheap sub fareclass) is not applicable to the flight you want. For instance, there are two flights a day HKG to SFO. It may be the case that V class is not valid on the 2:10pm departure, but only on the 5pm one.

3.) CX never sold V class in the first place for the flight. Sometimes during Chinese New Year, Christmas, etc. the only sub fareclasses ever available in Y are Y, B and H.

In each of the scenarios above, you cannot force CX to "create" the cheap sub fareclass for your flight, even if someone onboard the flight may have purchased it. With regards to #1 before, this is a mistake I've seen a GO colleague make before. Basically, he assumed he would be eligible for the cheapest sub fareclass that had been sold on the flight. He had seen a big discount a few months back on the flight he wanted (I think it was V class, since it accrued Asia Miles). Since he was GO, he decided to procrastinate until about a week before his flight. When he went to book using his GO seat guarantee, he was surprised when the fare was about HKD1,000 higher than the cheap promo fare - he was being quoted a higher sub fareclass, and was informed the V sub fareclass did not exist anymore. It wasn't that V class was sold out - as a GO, he was eliglble to force his way in if it was valid - but the V class fare he had seen advertised in SCMP had to be purchased before a certain date, and that date had lapsed by the time he was booking. It was like (example only here) "purchase before November 30, 2009 and fly before March 30, 2010". He did the equivalent of waiting until about March 20, 2010 to purchase, and then was surprised when there was no V available anymore. I think he ended up booking K.