As a deacon candidate going through formation, I have seen the benefits of Lay Ministry, since our program combines the training of Permanent Deacons (clergy) with Lay Ministers (people who are not ordained, by rather appointed). So no, I do not believe that it detracts from vocations to the priesthood.
Keep in mind, also, that Lay Ministry is important since a trained and appointed (by the bishop) Lay Minister can cover things like Marriage Counseling, RCIA, Religious Education, Music, Financial Administration, Parish Leadership when there is no priest available to the parish, etc. This then allows the priests, deacons, and even bishops to concentrate more on the clerical and sacramental roles that they do.
The official role of Lay Minister is a developing one in the post-Vatican II church, and how it goes remains to be seen. One thing that should not be done, though, in my opinion, is blur the lines too much between the callings of clerical ministry and lay ministry. For example, I believe that Lay Ministers should not wear robes or such. Keep a distinction between the two in the most obvious ways and the rest will fall into line.