Seems that the return email comments don't make much sense especially since you know that the person in question is not really at the company anymore.
Such as ON VACATION, WILL RETURN NEXT WEEK, PLEASE FORWARD ALL INFO TO (PERSON OF CHOICE). Why not just say .... sorry but we fired that person , got someone cheaper or really don't care.
Just sent a few to friends that are gone or on an "unpaid" vacation , not of their choice. be my guest ... go for it and while you are at it , call them (where they used to be).