VNAV during intercept course B737NG

[FL212]

Hi!
Could anyone explain me, what does VNAV show me (what it computes) when I am on downwind under radar vectors and I extend rwy centerline e.g. from FAF.

Thanks!
Adam

[Tom!]

http://www.pprune.org/tech-log/415623-737-vnav-profile-extended-centreline.html