He didn't have to respond "oh, I agree."  But since he bothered to respond at all, he could have said something along the lines of "it is a blessing to hear from people concerned about the future of our church.  The issue of vocations and how we can ensure the availability of the Eucharist concerns me greatly as well, and I am glad to hear the thoughts of my flock as I reflect and pray on this.  I hope you will also pray for other issues in our society, such as..."  Mentioning the issue in the petition would at least have demonstrated it was more than a form letter and that he might actually have read it.