The COVID-19 mRNA vaccines do not contribute to herd immunity because they do not confer immunity. They don’t prevent transmission nor infection, hence herd immunity is not attainable from these vaccines alone.
The key infectious strains we see today are variants that, in no small part due to increasing herd immunity to prior strains, are microevolutionarily driven to evolve around the vaccines. The same way the flu develops strains that bypass vaccine protections throughout the year.
Even with the different variants, the vaccines have reduced the transmissibility and infection rates vs nonvaccinated individuals, with some allowance in limited datasets (ie. Ottawa) for behavioral changes between vaccinators and nonvaccinators.
Not sure where you acquired your misinformation, but you may need better sources.
Review of Medical Microbiology and Immunology, 13th Edition. by Warren Levinson. Page 259.