Hey everyone! In response to a comment on the Ghost Sense page, I've decided to repost this little explanation/theory I created last year for a World Building Week event on Tumblr. It hopefully provides a good reason why we don't see Vlad's ghost sense, and how the sense works in detail (beyond what Frostbite tells us in UJ). Here it is:

We all know Danny’s ghost sense is a small release of energy from his ice core that triggers when a ghost is nearby (as is Dani’s, whose sense we can assume behaves exactly like Danny’s since she is his clone), but what about our other ghost hybrid, Vladdie?
We never see his ghost sense go off (as far as I can remember), but in TUE we see Dan’s escape through his nose as two red wisps of smoke. Since he is a fusion of Danny and Vlad, and we know Danny’s sense is blue, we can assume the red came from Vlad (and not from the evil-ness; shush this is an ectobiology class we deal with science).
Point being, if/when Vlad’s ghost sense goes off (perhaps he evolved out of it as he grew, or knows how to handle it better), it would be red, not blue. But Vlad does not have an ice core like Danny. If anything, I sometimes subscribe to the headcanon that Vlad has a fire core (which explains his pink-red plasma powers, instead of ice).
So the explanation for different ghost senses? I believe that both generally behave the same (i.e., are released as small bursts from the core when triggered by a nearby ghost). And the basis of all ghost senses are minuscule ectoplasmic particles (much like how we release moisture in our breaths as humans). The differences in color come as fusions of this “ectoplasmic breath” with the ghost’s core’s essence when the core is triggered and this is released as an altered breath, the ghost sense. Therefore, Danny’s sense/breath is blue from his ice core, and Vlad’s is red from his fire/plasma core. But both have the same basic properties.