REST API. Getting error posting new message

You're probably hitting a limit on the URL parameter length.

Try adding the message body as data to the post request, like this:

$.post(
   "/restapi/vc/boards/id/genericboard/messages/post",
   {
   "message.body": vBody,
   "message.subject": subject,
   "message.author": "/users/login/Per"
   } );