Are they giving free vaccination shots to dogs?

I'm not aware of free, but they do offer low cost vaccines

SPCA of Texas Commuity Wellness Event Mary Spencer Spay/Neuter & Wellness Clinic December 9th 8am until 12pm 4830 Village Fair drive Dallas, tx 75224 FREE Rabies & DHPP/FVRCP

If you get your dog there they will pay for your vaccinations and neutering and chips but not if you bring in your already owned animal. It's still pretty low cost if you bring in your animal.