Figured it out! Since I am new to Sigil I didn't know that a style sheet needed to be 'linked'. Now that I have linked the style sheet my text displays vertically in Sigil.
After conversion it also displayed correctly (i.e. vertically) in the Amazon Kindle Previewer - but only in Paperwhite mode ...
One caveat though is that in Sigil the text is vertical but the glyphs have not been rotated. It's not perfect but that's just in Sigil, the end product displays correctly on my Kindle Paperwhite.
For some reason though, on my computer, the text doesn't display vertically when using the Kindle software. Something else I need to look into
