VyOS command and configuration syntax remains compatible with Vyatta Core 6.6 system it was forked from. It means that old documentation is still relevant, for features that existed before the fork.
The last Vyatta Core release that had publicly released documentation is 6.5, but it's also still relevant. You can find copies of that documentation at:
Rewriting the old docs
Rewriting the old docs is the most straightforward way to improve the wiki.
But! Note that the old docs are not under a free license and, technically, not even redistributable. Do not copy anything from it verbatim. Use it as a reference and source of information, but write new documentation from scratch in your own words. We do not want a legal or moral trouble with the original authors and copyright owners.
Also, note that sometimes that documentation is not accurate. If something doesn't look right, it probably isn't, check on a test VM.