Make sure that Autosave is enabled. If Vivien or Ervin are using weapons like Two Handed Sword, Pike or Lances, they cant equip their shield. To do this have the shield you want any of them to equip in their respective inventory, don't have any other shield in their inventory. Now go to a different location (use the signpost). Now exit. Now while in the main menu, select Load Game, and load the Autosave that was saved automatically and at least one of them should have automatically equipped the shield. If it does not work try going to another location and repeating the steps, or drop the shield, go to another location, come back and get the shield and repeat the above steps again.