Why does my dark vileplume card state that it's weakness is fighting?? I
thought vileplume's weakness is fire?? Is it a misprinted card?

Well, this was a big error on WotC's part.  For some reason, they accidentally printed a fighting weakness on the non-holos and a fire weakness on the holos.  The error will not be corrected, and you may choose which Vileplume you wish to play with.  The value is not higher on either because the error will not be fixed.

Scott