Finding 2 points along a pline

I have plines of circuits that sometimes have T's (line goes along, then T's out then back to T point and then continues).

Here is example line -

(setq List_Pline (list (list 0 0) (list 1 1) (list 0.5 1.5) (list 1 1) (list 2 2) (list 3 3) (list 3.5 2.5) (list 3 3) (list 4 4)))

nth 0 = (0 0)
nth 1 = (1 1)     - T point
nth 2 = (0.5 1.5)
nth 3 = (1 1)     - T return point
nth 4 = (2 2)
nth 5 = (3 3)     - T point
nth 6 = (3.5 2.5)
nth 7 = (3 3)     - T return point
nth 8 = (4 4)

I need to be able to pick 2 points along the line and find the shortest route.

Examples - 

1st point at 1,1 and 2nd point at 3,3 - new line is nth's 3,4,5

1st point at 0,0 and 2nd point at 1,1 - new line is nth's 0,1

1st point at 2,2 and 2nd point at 3,3 - new line is nth's 4,5

1st point at 3,3 and 2nd point at 4,4 - new line is nth's 7,8

1st point at 0,0 and 2nd point at 3,3 - new line is nth's 0,1,2,3,4,5

1st point is 1,1 and 2nd point is 1,1 - new line is nth's 1,2,3

I've tried using vlax-curve-getParamAtPoint, but it returns 1st T point.

Synopsis of program -

Select Pline

Pick 1st point

check if on line

Pick 2nd point

check if on line

check if 2nd point is before 1st point, if so swap locations

find last nth of 1st point

find 1st nth of 2nd point

return list

Thanks in advance!

Alan