Specifically, the at-symbol as used e.g. on Twitter or Github (or many other sites besides), e.g. '@somename, what do you think about this?' or '@foobar, I'm waiting!' Comparing it with English/German 'o' and the Japanese よ (yo) (which are of course quite strongly marked), in the sentences above it would be possible to use these to similar effect. And while one might consider being notified on those platforms a similar effect to the extra emphasis the vocative puts on the addressee, @ing certainly also works and is used in places where it performs no other function but clearly marking the addressee. (@steph, do you want to grab dinner tonight?)
Are there reasons to not consider it a vocative marker?