It will also depend on the nature of the upgrade that you are seeking due to a way that VS work their mileage upgrades. If you are paying for Premium Economy and wanting Upper, it is fairly simple BUT if you are paying for Eco and upgrading to Upper then it gets more complicated. This info is based on making this kind of upgrade on VS three years ago, so the system may have changed.
When you book this {Eco~P.E.~Upper} then the system wants to see:
1) A seat in Y with a booking class that allows an upgrade.
2) A seat in P.E. with a booking class that allows an upgrade.
3) A seat in Upper that is available as 'Mileage'.
The fact that the first two seats will not be filled and will drop out of the booking and become available again is irrelevant! Since many folks want to go Y to P.E. these booking classes fill first and may block you getting into Upper. The reason behind introducing this extra 'hoop' will be self evident and, as I say, may no longer be applicable.
If you pay for P.E. and upgrade to Upper, there might be seats available as you only need items 2 + 3 from above! Once again, the seat in P.E. needs to be available before the upgrade can be made. I agree that waiting is high risk and, having been in the same bind before with a PE~Upper, I changed the outbound date and got the seats and routing that I wanted.
On that point, perhaps consider an Open-Jaws via SFO?? The cost of the internal flight is small or the sector can be driven by car in one very long day with two drivers or with an overnight stop and the scenery is good. I once included this mini-road trip in a longer journey and it turned out to be one of the best parts of the whole RTW!!!