It's a very good question you pose.
I do know that if you're on a base in HDSEL the VNAV will give you a completely different answer than if you have it in LNAV.
I've seen people "sneak a peek" by toggling LNAV when on a vector. That seems to jive with what gw_04 is saying.
Just so I'm on the same page: Assume you're on vectors 090HDG to RW36. Assume a "3-4-5 triangle" with a 3-mile base to intercept a 4-mile final to FI36. Your PPOS to FI36 from the base would be 5 miles.
If I understand gw_04, is saying VNAV would assume you have 4 miles to go and 900' high, if you're on profile. (Since it doesn't account for the 3-mile base.)
If what Capt S. is saying is correct, the VNAV would think you're going PPOS DIR to the FI36. You'd be 5 miles away on the base vector, so the VNAV would want you 1500' above FI36. To be on a 3-1 with a
really sharp 90-degree turn to final, you'd actually have 7 miles (3-mile base + 4-mile final) to go, or 2100'. It would show you as 600' high (2100' - 1500').
Now I've totally confused myself. . .
Like I said, good question.