Not wrong. Many if not most faiths include a strong element of extra-scriptural traditions of one form or another. Within Christianity, this was one of the original issues behind the schism between Protestantism and Catholicism, though nowadays many Protestant faiths have added on to their scriptural interpretations with tradition-based de facto doctrines of their own. Similar phenomena can be seen in most other faiths, and the scripture+tradition-based sects tend strongly toward being more popular than the scripture-only sects.
Why does this matter? Because for faiths like these, pointing out that there aren't find any anti-vaccination passages in scripture isn't a very compelling argument. They already get their doctrine partly (if not entirely) from other sources too, so there's plenty of room for antivax crap to slip in.