I think anecdotal evidence holds some value. For the parish that is using all boys and believes a correlation in vocations is causative, then I can see why the priest would want to keep that practice.
No. Vocations have not dropped, except as a percentage, and are still in keeping with what is happening in the rest of Christianity. The greater crisis is the secularism which makes Man rely on himself and dismiss both the need and the idea of God. I believe this to be the root of the decline of vocations in the West based on comparisons with the Church world wide and looking at other denominations.
Yet it remains that what you say must be done. Boys must be encouraged to seek serving the Church from the onset, as a lifestyle, to develop priests as role models and find a love of liturgy. Ditto girls.