If I remember right the Vikings never blocked a Dallas field goal. Their defensive TD was on a Crayton fumble. On that play, Crayton fumbled, and Ben Leber picked the ball up and ran with it, then pitched it to Cedric Griffin who scored the TD.
My best guess is that since the original player that recovered the fumble (Leber) was not the one to score the TD, that might be why your team didn't get the fantasy points for the TD. I would have thought the VIkings D would still be credited with a TD, but that's the best answer I could give you as to why you weren't awarded those points.
Otherwise you may be thinking of the play where Romo was sacked and fumbled, and Kevin Williams returned the fumble for a score, but there was holding during the fumble return and that brought the ball back to the 31 yard line where Minnesota retained possession but was not credited with a TD.
"90% of the game is half mental" - John Madden