It might be easier to make supports that you can insert (and maybe even move to allow a decorative effect). 1.25m is quite a span for vinyl. Try the sagulator to see if scaffold board would improve things in theory but supports could just be added to what you have there. You might need to add one inside the cupboard though.
That's a handy tool. Reckons I could get 100kg on a 24mm fir ply shelf before it sags even a mm, at that span, so I think I should be safe? Especially with an edging strip.
It might be easier to make supports that you can insert (and maybe even move to allow a decorative effect). 1.25m is quite a span for vinyl. Try the sagulator to see if scaffold board would improve things in theory but supports could just be added to what you have there. You might need to add one inside the cupboard though.