Both A-Frame and VRML use an XML based approach to describe the scene graph but the similarities stop there. As someone mentioned, A-Frame is an entity component system that makes extensibility very easy. A-Frame is also not a standard but a JS library allowing us to iterate on the API much quicker and based on real world usage. With the lessons learned we might want to consolidate the API in a standard in the future. jQuery and its crystallization on the querySelector API is a precedent of this approach
More than one is great—but ideally we'd have at least one which has reached a level of completeness, stability, desirability etc. to be a sort of standard people are happy with (again more than one such standard would be great). So if the tradeoff is completing a small amount or starting a larger amount, I'd rather have the small number of more complete options.
I guess if you're already happy with VRML, you're set though ;)
VRML was largely just 3D model format. WebVR is API-level access to VR hardware. WebVR frameworks are application frameworks that incorporate the ability to load many model formats, as well as provide different levels of configuration and convention.
