Vertical split of screen

I have a screen which I want to divide into three vertical areas. This I have done by using a vertical split, and sub splitting one of the vertical splits into another vertical split.

I want to be able to kick off an existing Natural program in the middle split, and have it executed within that split without removing the split to the left or to the right (almost like within a Subpage).

The existing Natural program is not available as a seperate Online function with a URL. Is it possible to Fetch Return this but have it execute within only a section of the screen?

No, this is not possible. There is a one-to-one correspondence between a page, a Natural session and a Natural program. That is: One page is handled by one Natural program that runs in one Natural session. The vertical split does not define a page on its own, like the SUBPAGE control does.