Why is it that even among traditionalists when the Rosary is said in public it is usually in the vernacular?
Traditionally there is no such creature as a traditionalist. There are only Catholics and non-Catholics (or "publics", as we used to call others when I was a child). In my memory the Rosary has been traditionally said in the vernacular. I have never seen it said in Latin and except once in a family setting and to me that seemed forced and unnatural.
This ignores my point that historically Catholics learned basic prayers in Latin and did a lot of their praying in Latin. Latin is more efficacious. Just like having the Mass be in Latin lets us know this is not an everyday activity, if you pray in Latin it may help you concentrate more and know you are not talking to your friend but to God. I am obviously not against people following along in the Missal. But again, if you go to Mass ever Sunday you will likely understand much of the Common in Latin
I wonder how old are you that you think that "historically Catholics learned basic prayers in Latin and did a lot of their praying in Latin."
Most, or many, Catholics learned their prayers at their Mother's knee, and not in Latin. There may be some who learned more formally, as in the schoolroom, but still they would have learned in the spontaneity of their mother tongue.
You seem to be confusing private/ personal/ family prayer with liturgical prayer. As long as prayer is from the heart it is good.