VLA (more properly CS.VLA - or Certification Specification for Very Light Aeroplanes) is a set of rules for certifying light aircraft up to 750kg. (There are other rules that you'd have to comply with as well, to do with manufacture, maintenance and so-on, but VLA is the biggie.)
It allows aircraft to be certified for issue of a Certificate of Airworthiness, although with light touch, the PFA/LAA also use it for permit aircraft.
It has however certain limitations. The biggie is that a VLA aircraft is limited to day-VFR only, non-icing conditions (paragraph VLA.1559(b) if anybody wants to look it up).
So, I'm afraid that this gets you not very far. The FAA do have a set of published requirements to allow night and IMC use, but that's only valid for an N reg aircraft.
Ultimately, for night and IMC use, a new aircraft needs to be substantially certified to CS.23, which is a whole new ballgame of certification cost and complexity.
Could it be done for a microlight? Yes, there are a few individuals or organisations with the skillset and qualification to do this - I'm probably one of them. It'll take a couple of years, and a lot of money: so ultimately nobody will try, and it's much cheaper and easier to just buy a CofA aircraft and have the required licence.
G