As an aside, I fail to see why anyone on a heading in the circumstances described (base leg or downwind vector with an extended centreline in the FMC) would be looking to use VNAV, or follow the vertical guidance.
By then I would ignore VNAV as in my opinion it is 'not an appropriate mode' for the phase of flight. I would be using V/S if low on profile or LVL/CHG if on or high on profile. As for working out the profile, I would be using my 3x table and mark one eyeball/ rough distance to go on my map.
By the way, I thought it did shortest distance to the extended line (i.e. 90 degrees). Will have a look on my next flight to see if it is this or direct to the active waypoint.
Cheers
PP