Make META DESCRIPTION the first words of the first posting

[abdobasha2004]

Any body can tell me how to do this without using vBSEO
It is on many forums
to show first 20 or 30 words as descreprion on top of showthread?
I want to put the first words of a post (thread) in the meta tags like vbseo do
but when I use $thread[preview] in any place other than threadbit it shows nothing
however on thread bit shoes the first words of the thread

plz help

important update :
Like vBSEO , how to put the first words of paragraph in the meta description of the showthread?
I found something and was very close, it is to use the value $thread[preview] that is defined in threadbit template ( I have a thread about this that no body answered)
The only thing it is not defined in headinclude template
so the question is : how to define $thread[preview] in the headinclude?
OR
Is there any way to put first 25 word or something in meta descirption not using vBSEO?

note: I asked folks at vbulletin.org even submitted a tickets in vbulletin.com support [the answer was so honest :Sorry I do not know what code modification are required to do this.]
nobody seemed to know how to define $thread[preview] in headinclude
isn't that strange?

*The mast important value for this that in gives higher ctr in search results (preferred by users) and remove short meta description errors in webmaster tools specially for short titled threads,