From what Tom said, they use Acetyl Proprionyl in VC v1. I don't get the stomach heaviness, though once VC steeps for 4-6 weeks, I can't vape it; the taste is as close to awful as it gets for me. Fresh, on the other hand, is perfect. I can't explain that one as normally I prefer slightly/moderately steeped e-liquids over fresh mixed since most of what I vape involves 5-10 flavors per mix.
If there's a night and day difference between the two, but still a buttery presence, the only other option would be acetoin that I'm aware of. Hopefully there's magic at work here and Capellas just put out a VC that doesn't have any of the 3 in there. Either way, I'll vape it for science

, at least once. If it doesn't turn in 4-6 weeks (or 8-12 hours of UC'ing / crock pot), then it may be a winner for me. If it steeps out like v1 though, I'll probably PIF the bottle to someone else.
I have about 12 4oz bottles of Capellas flavors on-hand right now and they're just sitting here lol.