Are you trying to claim an excerpted vanity is more acceptable than posting the entire vanity?
Concerning copyrights: Are you claiming a different copyright protection for your writing posted on a blog versus posted on your own FR thread?
Vanity is a posting without an external source and links, and wouldn't be considered a blog being a blog has its own web space.
If one posts on FR the entire work (as explained upthread) then you have a diluted copyright being it gives the owner of the site some rights to the work and you have no control over it the rights he has.
However if you excerpt the work and post it then the owner of the website does not have the completed work and only has a limited right to the portion excerpted. You retain complete control over the completed work which is desirable if you wish to sell the work later.