Basic usage principles explained
Unlike the version of Vocal Tune Studio integrated into Audio Evolution Mobile Studio, audio first needs to be recorded or imported into Vocal Tune Studio Standalone/AUv3 before it can be used to correct your pitch and timing. This doesn't mean that you have to perform your vocal performance live as you record within Vocal Tune Studio though (unless you want to of course!). In the standalone version you can import your previously recorded vocal performance and in the AUv3 plug-in, though you can import audio (see here), it is recommended that you simply record audio previously recorded, or imported, onto the track upon which the Vocal Tune Studio AUv3 is loaded as an effect in your host, directly into the Vocal Tune Studio plug-in. Multiple sections of audio from the host track can be recorded into the AUv3 plug-in this way and there can even be gaps between those sections between which the original audio on the track will be heard, unaffected by Vocal Tune Studio during playback. Audio must, though, first be recorded or imported into Vocal Tune Studio Standalone/AUv3 before it can be used. This is perhaps the most important thing you need to realize in order to learn how to use Vocal Tune Studio Standalone/AUv3 successfully.
The reason this has to happen is because, when Vocal Tune Studio is loaded as an AUv3 effect in your host, it can only, under the normal AUv3 effect setup, process audio in real time as it is played back in the host. The plug-in cannot directly access, or 'see', the audio clips on the host's timeline. This setup won't allow Vocal Tune Studio to fulfill its function though as it needs access to an actual audio file to be able to carry out, for example, time stretching. The solution to this problem is to internally (without any degradation) record the audio that is played back from the host track into Vocal Tune Studio - into Vocal Tune Studio Standalone/AUv3's own memory space on the device - from where it can then carry out all of the processing required for its full functionality to be available when the track is next played back in the host.
NOTE. Vocal Tune Studio Standalone/AUv3 actually records audio from the host track and both stores it as a file in its app folder on the device (standalone) - or in the folder specified by the user to be used for the AUv3 plug-in version (see here) - AND holds everything, the audio and pitch information at every sample, in memory (RAM) as it requires complete and undelayed access to the entire audio clip or clips. As such, it is worth being aware that Vocal Tune Studio can be quite memory intensive.
Please note that this process of 'recording into' Vocal Tune Studio does not have to happen with the native version integrated into Audio Evolution Mobile Studio (though it does apply to the AUv3 version if that were used within Audio Evolution Mobile Studio). There, the integrated Vocal Tune Studio can directly access and 'see' the audio clips on tracks without the extra step so it is this version which would be recommended for the very best workflow and user experience.