Generally, using VNAV all the way and keeping your head inside while being vectored is a depressing consequence of the modern way of teaching. You never see a chap or lass who has come up via turboprops, instructing or GA who do this. Because they can actually fly an aeroplane, not a computer game.
As stated above, the 737 FMC does it's calculations in reverse and once under vectors and human guidance from the ground, it isn't really that good at it. The further you deviate from the magenta line, the worse it gets, but constantly tapping away is just poor airmanship. Intuition and common sense should suffice, but it doesn't seem to anymore.
Finally,V/S and LVL CHG are mutually exclusive. Yo can have one or the other.......