If you install a font with support for the vietnamese and chinese charsets (UTF8?) - and can read the language

- then yes you'll be able to read ebooks in chinese and vietnamese.
One caveat though : the default font replacement trick wonderfully described by Valloric will work for LRF but will not work for ePub (which is read by Adobe Digital Editions, a totally different software)... You'll have to make some modifications to your ePub files (see my previous post)
Cheers,
David.