The question implies a loss of functionality if the parity bits aren't
written out to virtual memory. Actually, this isn't the case. Virtual
memory has no need to keep track of the RAM parity bits in parity-equipped
Macintosh computers.
The reason is that the parity bits are set when you write to RAM. When
virtual memory pages information back into RAM, the parity bits are set
once again as appropriate. Since the parity bits are for handling data
corruption in RAM, once you write to disk, it's up to the disk, its driver,
and the file system to ensure data integrity rather than using the same
mechanism used in "parity" RAM.
Also, when virtual memory reads the information in RAM, it doesn't have
access to the parity bits. It reads a byte; it gets that byte's value.
Finally, imagine the excess overhead associated with moving around and
storing that additional information. If you had 16MB set for virtual
memory, you would need an extra 2MB of disk space just to handle the parity
bits.