The vacuum diagrams are in the "engine control" chapter of the factory service manual for you vehicle. You would look under "VG33E/engine control systems" to find them. You can download the 2001 Frontier "EC" chapter from NissanHelp:
https://x.nissanhelp.com/forums/Knowledgebase/links/103/
I also attached some part diagrams, which might also help.
https://x.nissanhelp.com/forums/Knowledgebase/links/103/
I also attached some part diagrams, which might also help.