It is why Apple added anti-malware (with very few detection. See it in action in Sophos blog) in Snow Leopard.
You can use free ClamXav - http://www.clamxav.com/ or PC Tools iAntiVirus - http://www.iantivirus.com/ to scan your Mac OS.
>>>And can malware cross the partition from the Vista side of the machine?
Yesm especially if the malware is designed to work for both OS (example is OSX/Leap-A worm) and if the virus is clever enough to find the file share that you will enable (if you use such) in Mac to Windows using VM or when you transfer files to Windows system that is infected.
I've got a MacBook Pro running Snow Leopard. I've also gotten it partitioned so I can run Vista using the VM Fusionware. Is Snow Leopard susceptible to any viruses, phishing malware, or other spyware? And can malware cross the partition from the Vista side of the machine?