But we’re trying. 
Let’s try this. Instead of a whistle and an air column, think of a monochord. And let’s tune the string to C4. For a given string tension and mass, the sounding pitch of the string is proportional to its length. By definition, the fundamental of the string is the first vibrational mode, the pitch that sounds when the string is excited (plucked, bowed, etc.) in that mode - in this case, C4.
Now take your finger and very lightly touch the string at the halfway point, and excite the (entire) string. This is the same technique used to produce “harmonics” or “chime” tones on a guitar or violin. The note you hear is now C5, one octave above the fundamental. It’s the same note you’d hear if you actually stopped the string fully at the halfway point. But there’s a big difference.
When you stop the string, the half of the string that is not excited does not vibrate. It no longer participates in producing the pitch you hear. It is, in effect, as if you’ve created a new monochord half the length of the original. This is the equivalent of fingering or fretting the octave note on fiddle or guitar. Or of cutting your whistle in half, halving its length. And your new monochord has a new fundamental: C5.
By contrast, when you lightly touch the string, the entire string (both halves) continues to vibrate. You can confirm this several ways: by damping either half, which kills the note; or by videographing or strobing the string. The entire string is vibrating in its second mode, not fundamental. But the pitch is the same: C5.
Similarly, the fundamental of the whistle is determined by the entire length of the whistle’s air column (and its temperature and pressure and density). But when you play the vented D oxxxxx, it is the equivalent of lightly touching the monochord. The vent creates a low pressure point at the halfway point. And like the finger lightly on the string, it supresses any vibrational modes which would create a nodal maximum at that point.
But like the monochord, the whistle’s entire air column continues to participate in the note. Again, you can confirm this photographically or, more conveniently, by simply end-stopping the bell of the whistle - which will alter the pitch. If the vented D were a fundamental, then the lower half of the tube would not be vibrating, and any manipulation of holes below the vent hole would have no effect.
If anything, removing additional fingers would be expected to raise the pitch by decreasing the end-effect impedance. When in fact, removing additional fingers actually lowers the pitch - from the second vibrational mode (D) to the fundamental of the open-fingered tube (C#).
Which shows that the vented D is not the fundamental (and there is only one) of either the whistle’s full length or its shunted (open-fingered) length. Rather, the whole whistle is sounding its second vibrational mode, exactly as if overblowing the bell note D. The difference being that, like the lightly-touched monochord, the vent hole selectively suppresses odd modes of the fundamental, producing a different harmonic series. It’s the same series produced by fully stopping the monochord, or cutting the whistle in half. But what is happening physically and acoustically is quite different.
Whew! That’s too much brain exercise this early in the morning. But I hope that helps.
as far as I know). Half holing scares the bejeezus out of people for some reason, but it’s really not all that hard and it can also be used to play many other useful notes like Bb (XHOOOO),G# (XXHOOO),Fnatural (XXXHOO) etc. Do a search of this forum for half holing and you’ll see a ton of controversy. By all means give it a try.
sorry 
I’ll read it again when I have some leisure.
. Than k you.