get an 8 or 9 speed cassette that's slightly wider ratio than you want, remove a cog and spacer, fit the resulting shorter cassette on
Needs to be 8 speed cassette - to match the OP's STIs. Maybe need a 1mm spacer inside the large sprocket. On this cassette:
https://www.chainreactioncycles.com/shimano-sora-hg50-8-speed-road-cassette/rp-prod1864
13-26 - 13.14.15.17.19.21.23.26
with a 13t (outer) sprocket you can lose the 14t second smallest and still have a sensible gap going from 13t to 15t. Shifting may be slightly less smooth for that change (up).
As Sheldon says (edited): "[the larger] sprockets and spacers are in bolted or riveted assemblies"
Take care to check (before buying) that the spider has max 6 sprockets riveted together.
This Shimano site suggests that all 11t cassettes have 7 riveted sprockets.
Also take care when you do the indexing that you set the spare click on the 8 speed STIs on the smallest (of the 7) sprockets (so you'll need to shift two clicks to get onto the second smallest sprocket). If you put it the other end, there's a danger you'll damage the STI straining for one larger gear as the climb ticks up - and it'll be your fingers and the STI strength against the limit screw (guess which will 'win').
@TheDoctor 's suggestion is a better, safer option than straining the OP's lovely Vitus frame (I too have a 'screwed and glued ALAN) by trying repeatedly to shove a wide freehub into a frame the drop-out OLD of which is not designed for it.