To run MUI (multi user interface)you will need Vista Ultimate (which allows more than 1 language to be installed at the same time).
LIP(language interface pack) will change the language of say English Vista Home Premium to another Language.
Vista HP will only allow one language to be installed at one time, so either English OR Japanese - this is due to the license of this version.
(I refer to the dialog and menu languages, multilanguage text input / output can be installed on any version of windows)
But, as far as I know there is no official English LIP to install on Japanese Vista HP to change it to English...
my suggestion is to buy the Japanese PC with full English Vista installed, there are a few shops in Japan that will install fully functional English Vista on any Japanese computer, example :
QJue (Tokyo, Japan) Providing Computers in English, French, German, Spanish, Russian, Portugese, Arabic etc