i think i have found how to recreate issues people are having when using VST plugins with Vegas Pro.
I have tried it on several PC configurations (Win 10 x64 with nvidia and with amd too, Win 8.1 x64).
Vegas pro 16 trial, clean windows 10 x64 installed (patched), latest gfx drivers.
Tried with RME Hamerfall, NI KA6 and default m/b sound cards (Realtek). Same results with ASIO or MME.
Issues are bit different when using 32bit VST plugins, but lets start with 64bit VST:
64bit VST FX (ASIO or MME)
When i add any 64bit VST plugin and turn it on (lets take MAGIX eFX) to master/bus/fx/insert and move mouse over mixer (to be more precise any text boxes in mixer sections) audio stutters nasty. When i do zoom in/out (via mouse or via keyb, or via remote session just to exclude USB as cause) i get audio stutter. When i move window of any object same thing. When the 64bit VST is OFF, audio does not stutter, but when i repeat same actions that caused stutter while fx was on, vu meters either drop frame rate or freeze.
32bit VST FX
Situation with 32bit plugins is a bit different. On master/fx/bus there is less audio stuttering while on MME on insert almost none, but on ASIO it is unusable. Visual stuttering/dropped frame rate with happens same as i mentioned in case above.
I have tried all possible options to get rig of audio stuttering (buffers, tweaking nvidia drivers, GPU accell on/off, HT off/on, etc) but it just doesn't works.
Please fix this, i was about to buy Vegas pro (since it is on discount), but this issue makes it unusable for audio professionals.
I've used to work in studio where Vegas was used (long time ago, even before Sony aquired it) and it was dead stable and easy to use. Now seems it is unusable for any kind of audio post production. Please make Vegas amazing for audio production again (I really miss it!).
Issues seems to be with GUI vs VST server. Hope this video helps!