Polygon ColorPerVertex is not preserved properly when performing Edit Polygons -> Extract and Edit Polygons -> Separate commands. This makes it difficult to split objects that already have Color Per Vertex applied.
Note: As of version 2.5.1 of Maya, the problem with ColorPerVertex data loss for these two functions seems to have been addressed.
This script performs a long-hand approach to splitting an object apart: Dupe the object, delete the selected facets from the original and delete the opposite of the selected facets from the duplicate. This method preserves the Color Per Vertex information.
Select any number of face components. Faces may be selected on more than one object; in this case, a separation will be performed on all affected objects. Faces do not need to be selected in a contiguous block. If you select all faces for an object then no action is taken for that object.
Lastly, call this script. Each affected object will be separated into two objects. The newly created object will contain only the selected faces; the original object will contain all except the selected faces. The newly created object will be the same as the original with the addition of a numeric decoration.