Are there areas to avoid while flying formation with a larger aircraft?
In higher altitude the wake turbulence moves downward behind the aircraft. So as the pilot of the lighter aircraft behind the bigger you should keep the same level at least. 


photo by: Steve Morris, image source




  I suppose the wake turbulence only becomes dangerous at some distance, when the downwash forms vortices. – Sanchises


No.
The wake turbulence starts forming at about 2/3 of the wing span (measured from body). It starts as a real narrow but powerful vortex. Then it raises its diameter losing power (slowly).


photo by: Josef  Willems, image source

As you can see near the aircraft the  wake turbulence has a sharp profile and is quite predictable. Therefore there is a safe area directly behind the tanker.

related post: How does an aircraft form wake turbulence?