It hasn't been derailed. That was just never the issue.
The issue was maintaining the triangle between shoulder, fist, and elbow, by keeping the elbow at the lowest point, versus losing the triangle by allowing the elbow to rise up to above shoulder level.
VT punches keep the elbow low and drive forward from the elbow.
And by the way, whether or not and how much the elbow will rise in relation to the ground depends on how far you are extending forward or upward.
I can have my elbow one fist's distance from my body and extend an elbow-driven punch to a good three fists' distance without the elbow rising in relation to the ground.
Next time you go to a bar or have a tall enough countertop, table, or maybe dresser around, prop your elbow up on it in punch-ready position, fist face height. You might have to stand a bit diagonally to it, so you have say left lead leg, right arm punching.
Then, extend your punch not leading with the wrist and allowing the elbow to rise, but driving forward with the elbow down. See how far it slides on the flat surface without rising off. You'll probably get at least a good 20cm if you do it right.
That's punching with the elbow down. If you need to angle a punch more upward, the same low elbow applies. The elbow should not pop up and lose triangle even when rising in relation to the ground.