The Parallels/VMWare filesystem is just a disk image stored on the native filesystem (unless you use the Boot Camp partition).
Supposedly, the trigger for SSD/HDD is 3 accesses in the last three user sessions. (reboots, sleep/wake)
4GB of the SSD is reserved for real-time accesses/caching. The rest is available. Fusion Drive will use 100% of the available space on your SSD.