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Actually, Vim is ported to iOS, but it is not as useful, as Textastic.
This port is opensource, so may be it will help you in any way, to implement Vim modes — it would be sooo great!
Please, take a look at Pandoc:
it is very powerful markup-formats converter, which may be handy for such tasks. 
Also, online version may be useful: http://johnmacfarlane.net/pandoc/try