The example surfaces should at least be skewed at various angles to prevent HFrames from working. The Rhino 7 file (which I won’t be able to access in a few more days) has both surfaces and some edge curves (blue in image below), which further confuses the issue, because without the surfaces, it isn’t obvious which way the vectors should be directed…
Geometry rotated like this would be a better test:
Like this:
Vector_2020Dec17c.gh (16.7 KB)