Some observations about my 2,000-household parish regarding vocations:
We have had a boys-only corps of altar servers for at least the last 10 years; currently there are 49 servers, ranging from 6th grade through college. To have ten at the 11am Solemn Mass is not unusual.
We had one ordination in 2005.
We had another in 2007.
We currently have two seminarians.
Two of our college-age men (including my youngest son) will live in the diocesan discernment house next Fall. That's 2 out of a total of 8 for the whole diocese.
And of course there are also highschool-age servers who are explicitly considering vocations; one of our senior servers plans to go straight from highschool into the seminary.
I do not suggest a direct cause-and-effect here; nonetheless, the boys-only approach has a positive influence on vocations, and is a critical part of an overall conscious intent in the parish to nurture them.
The corps in action (scroll ahead to about 2:00)