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Unless you are a REALLY good programmer, there's no vertical split on MSX. The darn thing is just too slow... All you can do is split horizontally. Games like Herzog that *look* like they use a vertical split just redraw the entire screen, there's really no splitting being done. You can more or less do a vertical pallette split on MSX, but you'd need to use machine-specific timing (NOPs) which you prolly won't want to do in a BIOS.

You've got the two terms mixed up.
A vertical split is one that splits the vertical buildup of the screen, in other words a line split.
A horizontal split is one that splits the horizontal buildup of the screen, in other words a pixel split.

The vertical and horizontal 'direction' of splits, the way you seem to use the terms, is irrelevant, as a horizontal split (pixel split) does not have to be (and in practice never really is) a vertical straight line.
In fact if a MSX would be fast enough, it would be possible to create a horizontal split to split the screen in the diagonal direction. By your definition, that would be called a 'diagonal split', but in fact it's technically no different than a horizontal split, in proper terms.