In addition if you are using Xstream - by reading over at EON & various other places Xstream for 6 was pretty much a debacle/nightmare.

There is a huge difference between Xstream & the stand alone version.

My guess is ILM had a custom version of Vue - I can only base my opinion on this through reading the new maint. agreements EON has put out.  In one it states they will do customizations for additional$$.  Now if in fact they didn't have a custom version, keep in mind all they were doing was matte painting.....which rendering backgrounds of trees is really no big deal.