You can create and ROI like you want. Draw two triangles using Polygon, one inside the other (holding down the Ctrl key allows you to add another ROI to the image control). Read the ROI property to get the data for the ROI. There should be two contours listed in the order they were drawn. If the inner triangle was drawn second, then read out that contour and set the Contour Type (Contour ID) to 1 (Internal) and the write it back.
There is a bug present in LabVIEW that doesn't keep the Contour Type (Contour ID) value when you read it from the ROI property, but it does write it properly.
Triangle Outline.vi