In a VRRP configuration I've been using /32 subnet mask but I've never know the reason behind.
Sometimes I've using the same subnet mask as the primary interface, like /24, and in some cases it works and int others, until I set up /32, it doesn't.
For example, on Mikrotik documentation points to that, but without explanation:
Note: address on VRRP interface must have /32 netmask if address configured on VRRP is from the same subnet as on router's any other interface.
It's really a requirement? On FreeBSD CARP I didn't find a similar requirement.