Dunno about alternatives(on Vista) but did you try installing the X64 version in vista ? IIRC Phoenix Labs has released an x64 version.
Now although Vista x64 requires all drivers to be digitally signed u can use a software named "EasyBCD" . It has an option which allows you to install "Unsigned Drivers" on Vista X64.
Then try and install the X64 version of Peerguardian.
I am myself goin to instal it in a few hours.Will post here how it goes
When I installed the x64 version, I got the unsigned driver warning, (which I had supposedly fixed via cmd tweak "bcdedit /set loadoptions DDISABLE_INTEGRITY_CHECKS"). It loaded-up and updated...but I saw no action in the panel as I surfed, so I assumed it wasn't working, and un-installed. A story on Neowin.net said that they had a Vista version, but the link led to a 404!, and there was no news on Phoenix labs site. So I sit and wait and ponder...........