how does the rehearsemode work?

VST Connect is not a jamming tool. Rehearse mode merely overrides automatic disabling of Talkback during play. So you can coach the performer, but your Talkback mic signal is delayed against the playback for the performer by Remote Latency. Bottom line: no, when you sing along with the playback your singing will not be in sync for the Performer, as he is on a different time level. https://connectvst.com/ does explain this pretty clearly: “Keep in mind however that there is a long delay (as set by the Remote Latency parameter) which can be quite confusing. So for instance when you sing along to what you hear it will be significantly out of sync for the Performer.”