You lost me here. Vita instruments are instruments that respond to midi, not audio. If you have a keyboard connected, then you select a vita or vital instrument that will produce the sound for what you play on your keyboard. Thus, if you play an A-minor chord of A C E A on your keyboard, these notes will play in MMM using the instrument that you have selected. If you have a midi file already recorded that is not in A minor, you can transpose it, but if it is not already in a minor key, you would have to modify the midi file to make the notes correspond.
If you are talking about pre-recorded soundpools, most of which are audio files, not midi files, you cannot change the instrument to a vita instrument, or to any other instrument. It is an audio recording.
I presume that you have all of the Soundpools installed. Correct?
If you are totally lost, then you need to read the Quick start chapter in the manual. Follow along on the screen using a demo project while doing so. Look also at the tutorials on the Magix site.
In the media pool at the bottom of the screen under the Soundpools tab, you should have plenty of them there. If not, they are not downloaded and installed. Let me know what you see. Here is a Tutorial giving a Soundpool reference guide.
For the most part, the Soundpools are pre-recorded audio loops. Some are midi files. You can use the loops to create your own songs. See the demo for how Magix did one.
Many of the loops, other than drums, have been made in different keys. You select the loop that you want. To the left of the loop is a column of numbers, from 1 to 7, each having a different key. You then select the key that you want and then drag the loop onto the Arranger. Most of the keys are as in this Tutorial. There is another series that is slightly different. See this thread.
The instruments and synthesizers create sounds of instruments of one type or another. If you play an external keyboard or the one at the bottom of the screen and create a midi recording, you have to select an instrument or synth to get the sound. The Vita instruments and the others are thus for midi recordings.
Take a look at the above and then let me know how you make out. Indicate the version of MMM that you have, regular or Premium and the year. Things should get clearer.