I have had similar experience with Muv. In the evening I up it to 5 while not using the machine. On my 3.2 gig machine 5 works fine even with some internet activity. You cannot however stress the machine with real-time interrupt requirements while running 5 vacs. My network card and occasional very heavy disk activities have caused occasional pops at 5 vacs.
Interestingly even with 5 vacs running the CPU load (based on MS's primitive taskmon monitoring is actually still pretty low - about 25%). This number however is deceptive as it does not really measure a number of activities that the CPU and other MB resources are fully engaged in. I would not let the CPU utilization be your indication of success or failure.
I have asked for in a future version some way of monitoring when an interupt/dac conversion is missed so the user would know the created file might not be perfect. Something for a future release.
I have a suggestion to the guest. You can set the VAC to copy one of your recorded sessions to your audio out using the virtual audio cable; virtual repeater. If you are running 4 sessions then try using the repeater to your actual audio output and listen as you record (you only will get to listen to one session at a time). Then as an experiment try stressing your machine in various ways. You will hear when you have overtaxed the machine as the audio will pop or ssssddder. It seemed to be a good way to judge where and what can and cant be allowed while recording. I found my NIC card to be the worst culprit in my machine. For me it is my cheap NIC, heavy on CPU and interrupts.