Grow mesh vertically while adjusting vertex rotation?

I have probably been working too long, as this should be something rather simple…

I can displace verticis vertically independently of the mesh rotation just fine.
This normally works and doesn’t really need much of anything.
literally just feed 0,0,1 into the displacement will do it.

For what i need to do though, this is not enough.

I’m looking for the correct way to counter rotate each vertex based on the object rotation - so that bumping up Z on a rotated mesh doesn’t overly distort it.

I tried several things, and I believe the object rotation dot product to 0,0,1 should be the way to go about this, however I’m kinda lost as to how rotate each vertex by the appropriate measure.

I tried mucking about with rotare around axis a bit, but the results aren’t what I’m after.

To make the mesh grow vertically im using vertex paint and setting the lowest vertex to ignore the calculations.
Offsetting vertically stretches the area.
what I’m missing is the counter rotation…
Any ideas?