Elan gets in and uses an illusion spell to mimic the sound of the vampires and give them new orders? Elan has used major image before so I think it should work.

The illusion is not interacting with the people directly, and it has concentration effect so Elan can cast the spell outside and then maintain it via concentration.

So the only question is how does Elan get into the middle chamber past the first barrier which is Orange? How would it interact if Elan was polymorph prior to passing the barrier? The polymorph will be dispelled but will Elan be able to cross the barrier during this time or will it make it anti dwarf only effect kick in after the dispelled Elan?