VBS's role is to mirror the kernel and wall it off through a hypervisor. So your kernel/usermode can't access the secure version. This basically lets it compare the "secure" kernel and the regular kernel structures. Things like the process list, Driver executable regions, signatures, and such are mirrored. So when a process spawns and it's added to the process/threadlist. Those operations are mirrored in the secure kernel then randomly checked for security.
VBS also secures things like the scan timer/event and some other methods people used to use to disable it. http://uninformed.org/index.cgi?v=8&a=5&p=18 .
The performance impact shouldn't really be noticeable at all. All you have is some memory operations which are "Duplicated", but not really since COW. But i'm not that much of an expert on patchguard besides the really basic functions.