Good Morning JSP. Nico's response was based on my correspondence with him last week. Unfortunately, I was on the road and unable to test the details of the above process. The process will be a bit more complicated because I had overlooked the fact that woking as above would result in kins are the SC and CS points. Apologies to both you and Nico for that.
As you noted, the tools VPI alignment tool and the Arc Between Elements tool contain only SCS curve options and not SCCS.
I've tested the following workflow which should work. However, it does require a little work on your part to get a complete solution which is suitable for your project.
In the image below, the red lines are the original tangents. I left them untrimmed so you can see them better.
Then I used the Arc From Elements command to make a Spiral - Curve construct (in purple). Alternatively, you could use Spiral To Element command to get the same result. The difference in the two commands is that a FROM construct assumes that you know something about the beginning point (TS) whereas the TO construct assumes that you know something about the ned of radius (pass-thru point)
Then I used Arc Between Elements command to construct the Curve Spiral in green. This step could take a bit of patience since there are multiple solutions and getting the curve-spiral to fall into correct spot is sometimes tricky. In the sample below I assumed that I know the pass thru point of the end of purple curve, then I could get the correct green solution by snapping to the end of the purple.
As noted above, this workflow is not a one step and done like you would want, and it requires some extra thought on your part. But, this is the easiest solution I have found using the current tools. If this is something that you do a lot, then Nico can assist you in logging an enhancment request.
