what is WED error "vertex out of plane"?

See here:
http://www.conitec.net/beta/editorerrors.htm

and

http://manual.3dgamestudio.net/

for a complete overview.

A vertex can be out of the plane of its polygon when you moved it with the vertex manipulation tools. This can happen not with imported blocks, but with convex blocks that you made in WED. Polygons that are "bent" give this warning message.