Vishnu is always depicted as wearing the caste-mark of Vaishnavites. But I believe it represents Vishnu's foot-mark when worn by Vishnu's devotees, who show their devotion through it.
It has to mean something different when Vishnu wears it - since it is hard to imagine Vishnu putting His foot on His own forehead.
The Shaivite mark of ashes applies to Shiva Himself since all it represents is the ephemeral nature of existence.
