While Karkat is kind of a jerk, he’s not a full-on asshole, so he’s not going to tell anyone about Vriska’s message for as long as he lives. Not even those in his quadrants know about it. It’s Vriska and John’s thing and their’s only.
In fact, the only legit secrets the trolls are keeping from each other have to do with ~feelings~. A lot of them are cagey about their unrequited crushes (which normal 15-year-old wouldn’t be?!), though Nepeta has pretty much picked up on everything and added it to her new shipping wall—I mean, shipping paper pad.