If you vaccinate during a pandemic with an ineffective vaccine - you will force more mutations of the virus - much like bacteria developing antibiotic resistance over time.

Both delta and moronic show mutations in the spike protein, which is why the vax programmed t cells are not as effective in attacking these new variants. This, along with rapidly waning immunity of double vaxxed, is why vaxed are getting these new strains. However obviously the unvaxxed will still be at risk of getting these variants too.