have you seen the tv series? am i gonna be spoiling anything? i haven't read the manga.

It's set during mid-series I guess. Vash is already acquainted with Wolfwood, but wolfwood hasn't died yet (Also wolfwood has his second gun). Pretty sure it's just an original piece, with no relation to either the series or the manga besides the main cast of characters. Just think of it as a 90 minute stand alone ep.