Vertex colors will be preserved if you use a format which supports them (e.g. .OBJ, .FBX, .WRL, .BLEND, and point cloud formats). If vertex colors seem over-saturated or "washed out", try switching your color space between sRGB and linear in 3D Settings → Scene. By default, we choose sRGB.
Vertex Alpha is supported and can be enabled or disabled with the Use alpha toggle.
You can also improve color quality using Post-Processing Effects.