Do the vikings watching everybody die in "Death from Above" ever play any role?
I think they're just there to prevent you from going the route around most of the enemy base (i.e. you don't assault it from the southeast). IIRC there's even one message telling you that you should avoid the vikings (and one of the reasons you're given some Mutalisks a bit into the mission).
On a brutal play through they never made there way in, if you send overlords to them they will destroy them but won't make any move to destroy the base. They are effectively decoration. My guess would be a bug or someone overlooked them on the campaign design team.