I’m coming in now as I use Scrivener for editing Chinese – English and helping with English – Chinese translations. I’ve had other matters taking up my time over the last few days, so sorry I haven’t contributed before.
As you will have found out, Scrivener can display Chinese text while editing, but that is a whole project-editor-window view, so your English will also be displayed in vertical orientation; although you can split the editor, the setting applies to both editor panes. While you can edit in Vertical mode if you prefer, the outcome on compile will depend on what you subsequently open it in … see below.
That said, Scrivener is about getting the text down, in your case giving you features to make translating easier, and such questions as vertical layout for Chinese are really about page make-up, and are best left to post processing. While you may prefer to see the Chinese in Vertical mode, this is not essential to the translation stage, so ignore it for the moment. To elaborate on what KEWMS said:
- interleave the English and Chinese documents as she said;
- label each document as appropriate with “English” or “Chinese”, and set up collections based on those labels, so that you can have all of the English texts together in a collection and all of the Chinese in another;
- split the editor … for me a left-right split works better than a top-bottom split, but YMMV;
- open the English text in one split and the Chinese in the other, whichever way round suits you—the advantage of having English and Chinese collections is that they can be opened in Scrivenings mode if needed, and it’s easier to compile each language separately when you need to … if you don’t think you’ll ever need Scrivenings mode and don’t need the collections, you can still use the label as a filter at compile time;
- Until compile, Scrivener doesn’t actually know about pages, but if you turn on “View > Text Editing > Show Page View” for each editor pane—they have separate “Page Views”—you will have a rough idea of the pagination you will get when you compile
Then get on with translating and leave the vertical Chinese text till later.
If you compile to RTF, Nisus Writer Pro and Mellel 5 will only open Chinese in LTR mode; Pages will open it in Vertical mode, but in general, I find Pages plays havoc with footnotes, if you use them. If you compile to DOCX, NWP opens it in LTR mode, Pages opens it in Vertical mode and respects footnotes—though they’re on the left-hand margin and in Vertical mode too, so if your footnotes are in English …—Mellel 5, on the other hand, in my checking, won’t open it at all. I don’t have Word/LibreOffice/NeoOffice/OpenOffice, so I can’t comment on any of them.
On a further note, I find that Pages totally ignores the styles with respect to Vertically oriented Chinese in DOCX , changing the font, and the line and paragraph spacing, thereby also screwing up pagination—but not RTF so go figure!—but you can sort that out fairly easily.
Then when it comes to interleaving pages of Vertically oriented Chinese and LTR English, I can see two options.
[list=“1”][]Use a page make-up application. Sadly Affinity Publisher, which is affordable, doesn’t (yet?) have the option of Vertical text; that leaves InDesign or Quark Express, which I imagine must be able to do it, but both are very expensive, though a couple of months subscription to InDesign might be affordable if you could do the work in that time. There is also Scribus, I think, which I did install a few years ago, but didn’t need at the time, so didn’t really explore.
[]Compile the English and Chinese separately—using the labels makes this a doddle—either directly to PDF or to your word processor for tweaking and Print to PDF; then use a suitable PDF Editor to pull the pages from one PDF into the other, interleaving as you go—I guess Adobe Acrobat Pro can do it, certainly PDFPen (Pro?) from Smile Software can do it
[/list]
Which of those you choose depends on what is going to happen to it in publishing terms.
Hope that helps. 
Mark