It doesn't offer every vowel sound, but focuses on the most useable ones, which can be combined/filtered to get just about anything. If they went for every vowel, this set would have been 10+ CDs and over $1k, not just 4 CDs and $500, lol. Here's what it does have...
Full Choir - Ahs, Ohs, Oos, Mms, Oh<Ah Crescendo
Mens Choir - Ahs, Ohs, Oos, Ees, Oh<Ah Crescendo
Womens Choir - Ahs, Ohs, Oos, Mms, Oh<Ah Crescendo
Boys Choir - Ahs, Oos, Ees, Mms
Gregorian Choir - Ohs, Mms
Male/Female Stacks - Ahs, Ees, Oos, Ohs, Mms, Vvs, Zzs
Male/Female Scoops - Ahs, Ees, Oos, Ohs, Mms
Understand that these are all velocity switched, multisampled stereo sounds. And the above listings cover only 10 of the 29 categories offered on the four CDs. There are a lot of articulations, phrases and soloists in each category too.
For a full listing, go here...
http://www.ilio.com/spectrasonics/symphonyofvoices/index.html Scroll down to the bottom of the page, and the far right tabs offer detailed sound listings and a short exceprt from the liner notes.