My dogs are vaccinated and exposed to places where unvaccinated dogs surely frequent all the time.
This was my rationale. I've been accused of being overprotective a time or two, but this one isn't a dealbreaker for me.
The fact that this dog is so much smaller and that its owner has admitted to not doing a super job with dog socialization is more likely to be a dealbreaker. Those two factors seem like a possible starter kit for problems, but the lack of vaccinations...day in the park. Literally.