IMO it is not clear if the problem is the realtime algorithm that does the pitch shifting, or if the problem lies in the analysis part (which is not done in realtime as you know).
The Variaudio algo is licensed from zplane. Last documented (and audible) update in Variaudio audio quality was in Cubase 7.05, see here. So I assume the algo did not change since 2011, or at least not in an audible way. This doesn’t mean anything bad - why fix something that simply works?
Variaudio 2.0 and 3.0 just added much better handling and integration inside of Cubendo, but I bet the audio algo stayed the same since 2011.
Last updated docs by zplane are from 2021 and available here, and according to the docs, v3 didn’t change their SOLOIST/ TUNE algo (which probably is the one you can hear using Variaudio). The docs from v2 are here, so between 2011 (release of Cubase 7.05) and 2021, simply not much seems to have changed about the algorithm.
ProTools seems to use elastique v3 since last year. I did not find any information about Steinberg using v3 yet, so probably Cubendo 13 still use Elastique Pro v2.
zplane released a new product named INTONE lately, which is based on the soloist algo, but also includes the analysis part in realtime - now this would be interesting if it found its way into Cubase. Record in audio editor, pitch correct in realtime, and later edit the corrections if you like. Best of all worlds.
Lots of question marks. If the analysis part of Variaudio is the key to solve this, one would have to know if this is a Steinberg creation, or if it’s also part of elastique. Or simply ask zplane if noone of Steinberg gives any details.
Get yourself the demo app from this site (scroll down) and select soloist mode. Load an audio file and listen to stretch and pitch. That’s their 2024 algo. To my ears, it sounds exactly like Variaudio does today.