Yes, Latin was the vernacular in some countries. Vernacular, at least to me, means the primary language of a country or people, such as Spanish in Mexico or French in France. In that sense, Latin is no longer a vernacular language anywhere. Even if some learn it well enough to speak it.
I also would say that the only people I think it would be important enough to learn to speak it conversationally would be higher ranking prelates. Learning to read it would be good for anyone, of course.
Here is the section on Veterum Sapientia on its universality:
Universal
Since "every Church must assemble round the Roman Church,"8 and since the Supreme Pontiffs have "true episcopal power, ordinary and immediate, over each and every Church and each and every Pastor, as well as over the faithful"9 of every rite and language, it seems particularly desirable that the instrument of mutual communication be uniform and universal, especially between the Apostolic See and the Churches which use the same Latin rite.
When, therefore, the Roman Pontiffs wish to instruct the Catholic world, or when the Congregations of the Roman Curia handle matters or draw up decrees which concern the whole body of the faithful, they invariably make use of Latin, for this is a maternal voice acceptable to countless nations.
http://www.adoremus.org/VeterumSapientia.html
In other words, as Veterum Sapientia notes, Latin does not favor any nation the way a vernacular language would such as English. Further, another reason for the use of Latin is that the Church has “grown up” with it. As Fr. Hardon once noted, “God help the man who has to teach the Catholic Faith using English.” He said this because he noted that English has been influenced by Protestantism as some of the main countries that speak it are Protestant (the U.S., England). Latin has no such issues.
I don’t know why seminary training in Latin is “hard to defend.” Sounds like an excellent idea so Priests from different countries could more easily communicate. And read historical documents along with current Church documents in the original language. Of course, once the liturgy went primarily to the vernacular, that took a huge incentive away to learn Latin, no matter how much Pope John XXIII or Pope John Paul II advocated it for seminaries.
Good talking with you. God bless.