We considered writing the spec in the state machine vein, but I advocated for the declarative style. It may be worth rethinking that and rewriting it, essentially spelling out the parsing algorithm. As you suggest, a parallel document could be created for writers.
I'll need to study your spec further to see what the substantive differences are.
I'll be happy to open a post in talk.commonmark.org on the ambiguity problems caused by using a declarative style for the stmd spec. I'll do that once the forum is back (I can't seem to access it right now).
In parallel, I too will try to work out what the syntax differences are between stmd and vfmd. Meanwhile, please see: http://www.vfmd.org/differences/ (in case you haven't already).
Post on declarative style: http://talk.commonmark.org/t/571
Syntax diff: https://github.com/vfmd/vfmd-spec/wiki/commonmark-vs-vfmd