In my understanding, there is no interaction between bidi and vertical text. Whether you write rtl text top to bottom or bottom to top has nothing to do with bidi. When you mix rtl and ltr you would normally continue writing them in logical order when you write ttb or btt. Rtl and ltr are only meaningful in horizontal writing.
In theory, you could have bits of both rtl and ltr, embedded in vertical text, then you have the bidi problem again. Whether it is a problem partly depends on whether you want your glyphs rotated.
You might want to take a look at the directionality documentation for the Omega extensions of the TeX typesetting system, where they tried to come up with a coherent, universal directionality model that includes rotation and even mirroring of characters: http://omega.enstb.org/papers/directions.pdf .
Philipp
