VS Premium Economy Reward Upgrade Question

Apologies if this has been asked before!

I have booked and paid for in cash a flight for me and my partner, travelling in economy classic.
Am I correct in thinking that because I have paid in cash rather than miles, that I cannot use my Premium Economy Upgrade Reward voucher from my credit card on this flight?

Yes you are correct the upgrade vouchers can only be used on reward seat bookings. HOWEVER wouldn't harm to ring VS and discuss potential options. Also on the day of travel ask at the ticket desk and see what they say. Sometimes ticket desks do make weird decisions like this.

I once (2017) had them upgrade me using a PE upgrade voucher from paid economy and then miles to go to upper, when they shouldn't have done either. (was at JFK).

Never told them they shouldn't have ! Just played a bit dumb at the ticket desk and asked.