I like as a hobby to do text processing and analysis of CJKV text.
Japanese and Chinese is of course available in near infinite quantities.
Korean is a bit harder to find, but not impossible.
Vietnamese though has not been written in Han Characters for almost a century, and prior to that time literacy was not common.
I know there have been efforts to encode characters used only in Vietnamese and not the other languages into Unicode. But what I don't know is if there's been effort to encode old documents and make them available to the public online.