The idea was first mooted at the end of 2008, and in January 2009 I was asked to investigate the availability of scores and concluded that the most cost-effective solution, while not 100% perfect, was to obtain one online from CPDL. We tried out the first movement at about this time with some success.
There were (and are) editions there by four contributors. Only one of these (Mather) is a book of the complete work scored for choir and organ. The others are available as individual movements, of which one (Kickton) is available as full orchestral score and separate parts (so there's no one document scored for both choir and organ). Of the other two, the Ursic edition seems reasonably laid out but would require rather more paper than Mather, while Marmol/Casaus is quite compressed and not as readily readable. Moreover, the discussion page for this work on CPDL points out several places in both the latter editions which differ from other published sources. I chose the Mather edition as a reasonable compromise. The choral parts are reasonably clear, although the organ part occasionally suffers from being so compressed that the accidentals overlap with the previous notes.
So I printed and bound 23 copies of this edition with nice blue covers in February 2009. The project was then put on hold for several months while we concentrated on easy pieces, but in September our conductor asked me if I could obtain the second movement and was quite surprised when I told him I had the books ready. :-) And we've been working on it since then and performing small chunks of it, with the premier of the whole work taking place tomorrow. But if you miss it, there may well be another chance later in the year.
Now the Choralia web page tells us that there are two editions of Gloria published by Ricordi (edited by Casella and Malipiero respectively), which differ in various respects. The edition I've used has features from each of these (and, fortunately, no significant places in the choral parts where it differs from both), and by some chance it includes the five bars of music from Et in terra pax which are missing from the Casella edition. (What Choralia doesn't tell us, because it concentrates only on the choral movements, is that our edition also has 16 bars of music from the Soprano duet which are also missing from the Casella edition). In most other respects, our edition follows the Casella. This unfortunately means that people who come to us with their own copies will occasionally find their music differs from ours. Our occasional organist, who plays from his own OUP edition, needed to be advised of a couple of places where the harmonic structure differs (his opinion of one of them was that "Vivaldi probably didn't write that") — but fortunately, as his edition is similar to the Malipiero, he didn't need to be told about the extra five bars of music. The recording I have also mostly follows the Malipiero edition. On the other hand, when a guest singer joined us with a copy of the Casella edition, she suddenly looked bemused half way through the Et in terra pax movement! But that edition did agree with our copy on the bit that our organist disagreed with.
Which brings me to: why are there these two versions? (The OUP edition incidentally appears to have an A-sharp in a place where both the Ricordi editions and also the Naxos recording have A-natural, so where did that come from?) I don't know much about the history of this work, but I'd love to find out which one is the one that Vivaldi actually wrote (though that's most likely impossible from this distance in time).
(Another work that there are two versions of is Purcell's Bell anthem. The one on CPDL (coincidentally also contributed by Mather) is the same as the one that we have at Wesley Memorial in a rather old book of church anthems (even down to the remark "The original symphonies from the Purcell Society Edition may be played at this point"). I'd like to find a copy of the anthem from a source that satisfies Mutopia's definition of a public domain publication and see if it differs, though I'm not sure how to go about it. Of course, you'd think that being a member of a university with one of the most celebrated libraries in the country I'd be able to do that quite easily, but I'm only a poor computer scientist so what would I know?)
e And our guest pianist for today and tomorrow (who incidentally turns out to be the conductor of the Cherwell Singers) turned up with… the Casella edition. :-)