That's a poor measure of vaccine effectiveness, it's unadjusted and fails to take into account literally all variables as listed in the footnotes on that page. Vaccine effectiveness is shown in a different section.
But if we look at your observation that vaccinated people are testing positive at twice the rate of unvaccinated people it still shows vaccine effectiveness given that only 18% of the population are classed as unvaccinated (1 dose or none).
If the vaccines were ineffective we should be seeing a situation where unvaccinated people were testing positive at a rate 4.5 times less than vaccinated people rather than 2 times less frequently as you have observed.
So 'something' is causing unvaxxed people to test positive more than twice as often as you'd expect given their low representation in the general population.. That 'something' is their lack of vaccination. They must be getting infected, becoming ill enough to motivate them to be get tested (I would bet that unvaxxed people are less likely to get a test) and then testing positive like crazy to become so over represented in that table.
In other news, a hospital in cwmbran recently reported that every single one of its covid patients in ICU was neither vaccinated nor in a high risk age group.