Hmmm .... this is very interesting and raises a number of issues.
Firstly, your test image does not really prove much because there are no areas of fine detail and no large, plain areas
Vegas rendered the image in just 160 seconds but the movie duration was only 40 seconds and not 80.
The movie quality is far inferior to the AVISynth version, fine detail is blocky and large, plain areas are posterised.
The Vegas version is 33MB, the AVISynth version is 11MB !
When I created the Custom Template, the title was 'Custom Settings' not 'Custom Template'.
There is no mention of Profile, number of reference frames and deblock filter.
It does mention 'interleaving'.
Maybe the template settings available depend upon the type of source image ?