Yes, the studies are by CARA, Center for Applied Research of the Apostolate. At least some of the studies have been conducted on behalf of the bishops of the United States if not the majority of them. If you have questions as to who requested the studies, please contact CARA directly.
The information about Mass attendance is from several studies which CARA has done; the information is on their website or is available by searching, for example, using CARA Mass statistics, or CARA ordinations. Not all is immediately available on their website, but are reports which can be found individually.
As to any impact whatsoever, no study is ever going to answer that because it would require asking every boy who ever served whether or not he ever thought of a vocation, whether or not he ever served with girls, and what effect that had.
However, there is a clear set of statistics which CARA has: number of ordinations in various years (I have noted 2 - 1965 and 2014); and of Mass attendance in various years (I have noted both 1965 and 2014, including their breakdown of attendance by age group recently, particularly the age group of 18 to 30 and 31 to 43).
The statistics of birth rates in families is published elsewhere.
As to what the Church has stated, it is set out in the thread above; they have allowed girls to serve, and want boys to serve. Some are interpreting that as wanting only boys to serve; but Rome is eminently capable of saying that, by saying that girls may not serve. Since they did not say girls may not serve, then it would logically follow that they are saying that boys are not to be excluded.
There are apocryphal statements that boys are being excluded (with absolutely no evidence whatsoever), that boys are quitting serving because they don’t like girls (again, apocryphal, and an issue of parenting).
However, the only facts which have been shown have come from CARA studies, and in looking at Mass attendance between 1965 (about 50% ) and Mass attendance now (about 21% to 23%, so a little less than half as many), and ordinations in 1965 (994) and 2014 (494, or a hair over 49% as many), there simply is not the drop that would occur if girls were causing boys to not think about vocations.
Which is to say, there is no statistic shown that vocations have dropped as a percentage of families attending Mass. Clearly, vocations have dropped. But they parallel the drop in Mass attendance. That surprised me; I thought that it was worse.
CARA makes no bones about it; they note that while vocations have been edging up, it would take about 200 more per year simply to replace the priests who are retiring or dying. That is not good news.
Essentially it comes down to the fact that some people are in a range of not liking girls serving Mass, to being adamantly opposed to it; but none of them have shown any facts that it is having any impact whatsoever. They are the ones protesting that girls are somehow causing boys to not consider a vocation; then fine; let them get together, put up the money for a properly designed study, and show it - or not show it. Anything else is simply urban myth.
And the bottom line is that if girls serving Mass were having an impact on vocations, then the rate of vocations should be significantly lower, instead or paralleling the rate of Mass attendance.