Here are my folder sizes (includes a few installed drivers and programs but nothing big)
Program Files (x86) 337MB
Program Files 489MB
Windows 11.4GB+
Total ~ 12500MB = 12.2GB.
I have 2 GB of RAM. My default pagefile was 2GB I think, and by default hibernation was also on, which results in a 2GB file you may not need.
Anyway, for testing, if you can install Vista on the drive (i.e. the installer says you have enough space), you'll be fine.
Edit: The installer requires either ~14.3GB (~14100MB), or 14300MB which is a little more.
The above poster is probably right though, 15GB (using powers of ten) comes out to 13.96GB (powers of two) which is probably not enough space to even install, unless you have some way of getting around that.