So, to continue, and conclude, this fascinating conversation with myself, I'm happy to report that I've resolved the issue regarding the offset and rotation quirks, and it was, in fact due to the fact that I was trying to set the Vector2 Circle with Vector3 coordinates.
While this can't currently be resolved in PlayMaker without modifying the MakeCircle action (because there is no Vector2/Vector3 option, and Vector3.up is not exposed), I did successfully modify the Vectrosity script, which I've included below for any interested parties. Commented lines are the original "Vector2" version of the script.
Cheers.
#pragma strict
import Vectrosity;
var segments = 16;
var lineMaterial : Material;
var radius = 2.0;
function Start () {
//var linePoints = new Vector2[segments+1];
//var line = new VectorLine("Line", linePoints, lineMaterial, 3.0, LineType.Continuous);
//line.MakeCircle (Vector2(2,0), radius);
//line.Draw();
var linePoints = new Vector3[segments+1];
var line = new VectorLine("Line", linePoints, lineMaterial, 0.5, LineType.Continuous);
line.MakeCircle (Vector3.zero, Vector3.up, radius);
line.Draw3D();
}
