Why was the VC10 unusually prone to reverser-induced buffeting?
It may just be that other airframes mount their engines with sufficient clearance to avoid the buffeting issues that were seen on the early VC-10's. Ultimately it only took of a move of 11 inches to solve the issue 


  ...testing was prolonged by the need to move each engine pair 11 in (27
  cm) outboard. This major redesign was needed to resolve tail-plane
  buffeting and fatigue issues due to thrust reverser operation. The two
  inboard engines could have thrust reversers installed at last,
  matching the 707.


So while it may appear in photos that other aircraft have similarly mounted engines you would need to look at actual technical drawings to truly compare mounting distances. 

Its also worth noting that there is nothing saying other aircraft do not also see buffeting from their aft mounted thrust reversers, however they may not see enough to cause structural issues as was the case with the VC-10.