"New version with WordBuilder inside the included PLAY software. Imagine typing words into your computer and hearing a choir sing them in any key(s) you play".

http://www.soundsonline.com/Symphonic-Choirs

Maybe one of those : https://8dio.com/instrument-category/choirs/

DonnyThompson, post: 443364, member: 46114 wrote: "New version with WordBuilder inside the included PLAY software. Imagine typing words into your computer and hearing a choir sing them in any key(s) you play".

http://www.soundsonline.com/Symphonic-Choirs

Addendum: I need to be clear here, that I've never used this. I'm just going by what you said you wanted to do, and I typed it into Google, and the VSTi I linked to above came up high in the search returns.
I have no idea of the quality; it doesn't appear that there is a trial version available for DL, although there are audio samples for you to listen to on the site.

-d.

Prahas David Nafissian, post: 443363, member: 47544 wrote: Hi,

Anyone know of a virtual instrument plug in which creates choir sounds
(SATB) (or I'd settle for solo voices) which allows the user to specify
vowel sounds and starting/ending consonants -- in other words, allows
the user to create sung words?

Thanks!

I'm the small fish in this big pond, so TIFWIW, but I don't know of anything that specific. PT has a few different vowel combos, and vocal ranges. Eg. Male, Female, Ooh Aah, Eeeh Ahh etc. But you can't make words.

The Korg Kaosillator (sp?) is the only thing I can think of that might possibly work. But you have to record yourself, or import a sample to work with. I don't know if it does a choir sound either, it does a lot of funky stuff though. Might be worth checking out if it's a crucial part of the project.

Maybe a choir patch along with a vocoder would be something to consider.

https://www.freesound.org/people/shawshank73/sounds/122780/

Maybe this:
https://soundiron.com/products/olympus-symphonic-choir

Soundiron is proud to present the absolute pinnacle of professional choral sampling: The Olympus Symphonic Choral Collection. The Olympus Collection is a combination of our massive Mars Men's and Venus Women's Choirs, bundled together as a complete set. Olympus Collection represents the most complete and detailed choir sample library available anywhere, with sonic power and playability that cannot be matched. Olympus offers a superior range of detailed articulations, robust sonic presence and excellent true legato fluidity, all at an incredible price.

This collection covers a wide range of Latin, Russian (Slavonic) and Pure Vowel content, with a full set of chromatic articulations including sustains, marcatos, staccatos, true legato vowels, clusters, falls, swells, effects and a trio of stellar male soloists. In addition to the ensemble sections, you'll also find an excellent selection of 8 soloists for each standard vocal range: bass, tenor, alto and soprano. An of course, in addition to the comprehensive selection of traditional choral articulations, you'll find a wealth of useful FX and atmospheric content that can be often vital for modern music production and underscoring.

Working with Maestro Robert Geary, we recorded over 20 days at Montclair Presbyterian, high in the forested hills overlooking the San Francisco Bay, with a 63-voice hand-picked group of SF Symphony Chorus, SF Opera, SF Choral Society and Volti singers. Both Mars and Venus include a pair of wide stereo mic positions (stage/close and hall/far). These two master positions were carefully balanced and mixed using an array of 12 large diaphragm microphones set throughout the hall. This simplified routing is designed to give you the flexibility to incorporate the full-bodied sound easily and efficiently into your stereo or surround mix.

Also, if you find something for this, I'd love to hear the results.