You would need to phone VH to check - if there are only a few seats left with VS itself they will all be full fare (W) seats, so it is highly unlikely that VH will be able to offer any further seats at their discounted upgrade price.
Best thing you can do is now ask on the day, if the plane is full the upgrade price is likely to be discounted down as they would rather take some amount of money than be forced to do operational (free) upgrades, this is where you could also offer to use miles.
The thign I forgot to mention in my earlier post, was that the normal typical mileage upgrade are only available to the top 3 tiers of booking class for Economy (L, B and Y) as they don't really want peopel buying ultra-cheap N,X, Q etc fares and then using a bargain 10,000 miles to shift into PE.
Thanks,
Richard