The general rule when booking award seats, especially international ones in Premium cabin is as follows: advanced booking window, specific date of travel, or lowest points, choose two of three. In other words, if you want to travel on a specific day and don't bother booking in advance, you won't be able to book it for the lowest number of points. If you book well in advance (i.e. 6 months or more), you'll often nab the award for the lowest number of points. If you are completely flexible on the travel date you can often find a cheap award in the last minute.
One thing to keep in mind with award seats is it's different than a cash booking. In a cash booking, you can generally book whatever flight you want so long as there is a physical seat available in the cabin you're interested in booking. With award bookings, you are taking advantage of
excess seat capacity the partner airline has. These excess seats are often released many months (up to a year) before scheduled departure and are hence nabbed many months in advanced by frequent flyers who are flexible enough to commit to travelling on specific dates. In addition, airlines will sometimes release further seats to award programs when they can tell that the seat won't likely be sold. After all, it's better to sell the seat for points than to leave it unoccupied and earning 0 revenue. Lastly, realize that for international travel, you are relying on partner award availability since Virgin doesn't really have international routes. Partner airlines often tend to release less award inventory to members of other frequent flyers than members of their own program.
I suspect if you ask anyone who has made meaningful use of their points to travel international long haul in a premium, they will tell you that this isn't work for the faint of hearted. It requires experimenting with dates of travel, routings, and other such factors. For instance, I may not be able to fly Sydney to Singapore in J on Singapore using points but may find availability on Adelaide to Singapore or Melbourne to Singapore. Indeed, some people can't be bothered with such drama and use third-party services to facilitate the award booking for them, such as
Award Flight assist.
-RooFlyer88