I believe all versions of Vista, or at least Vista Ultimata, come with both x64 and x86, so you shouldn't have to worry.
There are so many things wrong with that sentence, yet there is nothing wrong, let me explain... First of all, you are correct, just not in the way you think..
To make this clear x64 IS x86. x64 is an extension to x86.
Every single version of windows is made for x86, so don't think that an x64 windows version wont run on an x86 processor. The x64 version won't run on x86 processors that do not support the x64 extensions.
The distinction you should be making here is
windows vista x64 (64-bit) and
windows vista (32-bit). There are 32-bit and 64-bit versions of windows, both of witch are made for the x86 processor.
To answer your question, your computer can run either the 64-bit (x64) or the 32-bit version, but you are much better off getting the 32-bit (non x64 version) version.
Edit: I forgot to add, the 32-bit and 64-bit versions of windows are sold separately, not together as the quoted statement implies.