What version of LANDesk Antivirus are you using, and what patches do you have applied?
LD Antivirus Version: 184.108.40.206
I'm writing this from a Vista-64 machine (yes I'm eating our own dogfood, running LDAV on my home computers) and right-click / scan for viruses works for me.
I'm on 8.8 SP3 but I seem to remember it worked before.
You will remember that "enable right-click scan" is an agent behaviour setting? Like me, you may well have deployed a different agent behaviour to your Vista64 machines because, until LD88-Spyware-20722-882 came along, you could run into trouble if you didn't.
That is the firs thing I checked, As a matter of fact if I install the same config on a Vista 32 bit PC it works fine.