Ok. Got it working.
New-UDMapVectorLayer -FillColor "black" -FillOpacity 100 -opacity 100 -weight 50 -Polyline -Positions @(
@("37", "-75");
@("38", "-76")
)
@adam, can this be added to the documentation for future people?
EDIT:
Found a fork of old UD documentation that specifically shows it should be an array of arrays.
The current help/documentation just says it’s an object