The vicarious humanity of Jesus
To speak of Jesus' "vicarious humanity" is NOT to say that Jesus is anything less than fully human. Scripture declares that the eternal Son of God became human through his incarnation, and remains human forever (see 1Tim. 2:5). The resurrected, ascended Jesus is fully God and fully human (now glorified in his humanity). And the one who will return in glory will be fully God and fully human.
Jesus is the permanent union of God and humanity in his own person: one person with two natures. Thus to say that Jesus is the "vicarious human" is not to suggest that he is anything less than fully human. Rather it is a statement concerning the meaning of his humanity for the benefit of all humanity. Because Jesus in his divinity, is humankind's Creator and Sustainer, his humanity has profound import for all people everywhere in all times. In his humanity he is the unique representative of and substitute ("stand in") for all humanity. This is what we mean by referring to Jesus as the "vicarious human."
Here's what this means: what happened to Jesus in his humanity, happened to all of us. When Jesus (who became sin for us) died to sin, we all died to sin. When he rose victorious from the grave, we all experienced in him victory over death and sin. When the man (resurrected and glorified) Jesus ascended to heaven, we all ascended with and in him (Eph. 2:5-6).
Paul says that the lives of all humans are "hidden" in the life of Jesus (Col. 3:3) - we don't now fully see who we are in him, but one day we shall. And forever our lives will remain in him, because forever he remains human - the vicarious human - God with us and for us, as one of us - the one for the many, the many in the one.