From a quick web search, VAIL was supposed to only be used as a fallback for when RAIL (i.e. a cloud streaming service for your Win32 apps) wasn’t available. So I assume it didn’t necessarily have to be “good” — it was the “safe mode” of Win32 compatibility, per se. It just had to be there as a rare last resort.
But was RAIL any good? Did it even get built?