Why do you want to Build it yourself, When you can get a pre-built image.
You can try KRescue Here. If 16.04 img is still available.
Try and build it yourself then follow this. But it will take some extra efforts to move the source from 18.04 to 16.04
https://docs.khadas.com/vim3/FenixScript.html
BTW Why do you need 16.04 as 18.04 is already LTS and stable?
UPDATE: Sorry looks like they dont maintain 16.04 for Vim3 anymore. So only option is to use fenix script but need to find all the reference to 18.04 and change it to 16.04 sources.
IDK if that make sense.