Operating system: Windows-7-6.1.7601-SP1 64 Bits
Graphics card: GeForce GTX 660 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 445.87
Broken: version: 2.90.0 Alpha, branch: master, commit date: 2020-06-24 14:01, hash: rB6fec2e4db05f
Caused by rBf7bbc7cdbb6c: Sculpt Vertex Colors: Initial implementation
Short description of error
If old vertex color exist and new sculpt vertex colors is not, I think, viewport should take first one until full replacement.
As far as addons will import vcolors to regular vertex color, it may be handy too.
I am not sure if it is a bug, may be it is intended.