[Vuejs]-How to use finished callback function in nativescript-texttospeech?

Try wrapping your own promise, that should help

talk: function(message) {
  var speakOptions = { ...this.speakoptions, text: message };
  return new Promise((resolve, reject) => {
     speakOptions.finishedCallback = resolve;
     TTS.speak(speakOptions)
  });
}