First of all,
Logic Pro doesn't use VSTs.... That is the a different sort of plugin type used by Cubase for example...
Logic Pro uses AudioUnits (AU) Plugins and Logic Pro X only uses 64bit AU Plugins...
Okay.. so with that out the way....
In my opinion, the best Choir type sounds are found via Kontakt Libraries (along with many other high quality types of sounds) so the investment in the full version of the Kontakt 5 AU plugin is well worth the expense if you really want high quality sounds available to you in the future...
Depending on the type of Choir sounds you are seeking.... as you mentioned 8Dio's Liberis ECC is very good... but the following links may give you some idea of the many alternatives out there...
http://forum.cakewalk.com/Best-Vocal-Libraries-for-Kontakt-m3007175.aspx
http://www.strezov-sampling.com/products/view/STORM-CHOIR.html
http://soundiron.com/products/mars-symphonic-mens-choir
http://soundiron.com/products/venus-symphonic-womens-choir
http://cinesamples.com/product/voxos-epic-choirs
...and finally a personal favorite of mine... It's one of the older sets and is much more useful for Choir "pad" type sounds but the quality is still right up there, at the very top.. for such uses...
http://www.spectrasonics.net/products/legacy/symphonyofvoices.php
While SOV isn't specifically for Kontakt, it's available in Akai format which can be imported and used by both Kontakt and Logic Pro's own EXS 24 Sampler....