The following are online citations from Moshe Chafetz's 1710 work Melekhet Machashevet, found on the internet.
Not all links are still active, but relevant text is excerpted here in full.
unconfirmed that this is in Vayechi commentary
"Yosef saw that his father was placing his right hand on Efraim's head and it was not good in his eyes. And Yosef said to his father, "Not so, for this (Yosef's older son Menashe) is the firstborn... but his father refused saying, "I knew my son, I knew... but the younger son will be greater"."
This is not the first time that Yaakov had favoured a younger child. Rabbi Moshe Chefetz, in his Meleches Machshseves, sees Yaakov doing the exact same thing, years earlier, when he shows favouritism to his younger son, Yosef. The envy and the angst that followed in the wake of that favouritism had been obvious to Yaakov.
For more information: The Chafetz Project
Any and all information about Melekhet Machashevet or Moshe Chafetz is greatly appreciated.
Contact me at djbrook@sbcglobal.net.