Could you book economy using your voucher and then upgrade via avios?

Also I’ve found them very flexible with using the voucher. What do the Ts&Cs say? If they don’t say you can’t use them for what you want to do, then I’d argue you should be able to.

I managed to get them to retrospectively apply a voucher for a previous flight, although admittedly this was because it had to be booked urgently and I contacted them ASAP after the flight had been taken.