I was thinking if its possible to put for example Vista (English version) and Vista (Russian version), both 32-bit editions, on the same installation DVD? Like, when I boot it up I can choose from the custom multi-boot menu which one to install. Can it be done or is there enough space for this on the single layer DVD?

You can easily integrate languages packs with vlite.Setup asking before installation.Its simple

I didn't mean Language Interface Packs.

I want to compose a DVD, where user can choose between Russian or English Vista installation.

Something like this, when booting from DVD:

----------------------------------------
Windows Vista install DVD
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1) install English version
2) install Russian version


Choose your Windows version: ...
----------------------------------------

The files in Vista itself are language-neutral (even English is a MUI), so this shouldn't be too hard. Documentation on how to do this (I believe you must be using Vista Business, Enterprise, or Ultimate though) is here, as other versions of Vista don't support MUI packs (someone can correct me if I'm wrong, but I believe you need one of those three versions to do this).

Cluberti you are right,

using one of the 3 mention versions above you can intergrate what ever language layer you want for Vista onto a DVD and the user can choose what version they want from the installation menu

At the moment I already have Vista English OEM installation DVD and Vista Russian OEM installation DVD