Welcome to Australia's leading independent Frequent Flyer and Travel Resource since 1998!Our site contains tons of information that will improve your travel experience.Joining AFF is fast, simple & absolutely free - register now and take immediate advantage of these great BENEFITS. Once registered, this box will disappear. And you will see fewer advertisements :)
I'm going to assume you've booked a cash ticket, that is a flight operated by Singapore airlines but with a VA flight number.
In this case the only path to upgrade is the full fare difference. This is the fare difference from the price you paid to the current VA codeshare price (in the higher cabin) plus the change fee. Often the VA codeshares will have less availability than the SQ flight numbers so even if a seat is available it might not be possible to upgrade.
The easiest way to see the cost and figure out if space is available is to search on the Virgin website for your flights (exact same numbers/dates) but in Business class. Take the price of that ticket and subtract the amount your ticket was. You'll also need to add the change fee ($200 in cheaper economy fares and free in Freedom fares).
If you've booked using Velocity Points as a redemption ticket then the only way to upgrade is to pay the change fee and the additional points for the higher cabin (pending award availability).
Other methods such as Bid Upgrade and upgrading using points/miles are only available on Cash or Redemption tickets that have been ticketed by SQ.