When do I attack if complete my battle line with a card stolen by Venemous Creeps?
From the Venemous Creeps section of the Chaos Marauders card reference:


  If a player completes a battle line with a stolen card, he may still
  attack an opponent’s battle line as normal.


So the normal rules apply, that is to say: "As soon as a battle line has been completed, it can attack any opponent’s incomplete battle line" [assuming the completed battle line is strong enough].  

Therefore, yes, you do attack on the current turn, not the next turn.