In the geo spreadsheet I see a variable “Map Offset”. How can I use this in vex ?

See the screenshot.

I'm not sure, but maybe with an underscore? “Map_Offset” ?

That's not actually a real attribute. It's the offset of the point, which is sometimes different than the index of the point (which is @ptnum). Offsets allow points to be deleted in the middle of model without having to resize every array.