It was always my opinion that Trask wasn't terribly concerned with finding the actual witch or ridding the house of evil - he was mainly concerned with persecuting someone.  With all of the Trasks, I always got the impression that the whole religion thing was somewhat of a sham.  He got power and position by doing what he did, and it's much more efficient to persecute someone who's already been named than to conduct an actual investigation.  That would take time, there would be doubt as to the guilt of the accused (not so with Vicki as Abbigail and the Countess were already convinced of her guilt), and there's always the possibility that the actual witch might have been a member of the Collins family - now that wouldn't have done at all for Trask's reputation.  Much better to persecute a lowly servant.