I know one example can’t prove anything, but it’s interesting that the diocese that has the greatest percentage of vocations per capita by far (Lincoln, NE) is one of the only two dioceses in the USA that doesn’t allow altar girls. Again, not proof of anything, just an observation.
I also know that our small parish, which does not allow altar girls, just had one priest ordained this year with 4 more young men in the seminary to be ordained over the next few years. In contrast, the entire archdiocese of Omaha has about the same number of priestly ordinations coming up even though there are dozens of those parishes, most of them much larger than our own. A year or two ago, they had 0 ordinations to the priesthood and this year they had 1.
Of course altar girls are not the only reason this is happening, I am sure, but I don’t think their use can be completely dismissed either.