After difficult research I've found the answer to my own questions.
I don't like it, and I'm not sure it's complete nor accurate, but I post it here to avoid to other interested people the same time consuming steps. Any amendments/additions are welcome.
1. The beta release of Vista (RC1) was freely available from MS as a download of 2.62 GB!

It's no longer available at MS, as "The Customer Preview Program is now closed". Don't know if it's still available somewhere else.
2. It came in a new image format (.WIM), that apparently can only be extracted with the utility Imagex.exe, wich was NOT included in the download (may be to keep the size of download small).
3. This utility can easily be obtained by downloading AWAIK, a free, small (in MS standards) 829MB file. AWAIK comes in .img format that can therefore be easily extracted (strange, they didn't think of releasing it in .wim format). My first download attempt froze at 99%, but the second went well.
4. This AWAIK is apparently needed to install Vista (why isn't Vista smart enough to install itself?), but is too small to include Vista, which is still needed separately.
To access the Imagex.exe utility, necessary to unpack the Vista image one needs to install AWAIK.
It contains several different versions for different CPUs, only one of which needs to be installed (great!).
I couldn't install it yet, as it complained that it wants SP2 at least (I was running SP1 on that PC).
It also includes other tools and utilities, included WinPE.cab (no need to download it twice!)
5. WinPE.cab contains two .wim images, and not a single word of documentation, so Imagex.exe is needed to see what's inside.
It's a 326MB file, so I assume it's the small MS alternative to BartPE or WinBuilder, but I'll have to check.
edborg