What we need for certain, other than git and build-essential, assuming the base system is minimal install:

live-build, python-pip, devscripts (that's where debuild comes from), sudo, real vim (as opposed to vim-tiny that may come with minimal setups), nano for people who hate vim, mc for people who can't get over two panel file managers, aptitude (apart from apt-get), Ada toolchain (gnat, gprbuild).

I guess if we have this, we may start using a VM deployed from that template and see what else is missing.

I made a thing that'll generate for you the development VM I'm currently using: https://github.com/giandvd/vyos-dev-vm

You can then run build-vyos and it'll make a VyOS ISO which you can test in QEMU from within the VM.

I think it's a great starting point.