First of all, what is a vibrato? Well, imagine if you hear a person holds a note while singing, you noramlly don't hear that person just holds that note straight, you hear some "vibrations" in his/her voice while he/she sings. Those vibrations are what we called "vibrato".
For those who sings or plays instruments that sustain notes, you all know that vibrato makes the tone of the voice/instrument richer and more expressive. That's the same case for melodica as melodica is an instrument that sustain notes; plus since melodica mostly is cheaply made, the tone of melodica simply sounds like a toy without vibrato. BUT, one of the coolest thing is that melodica is the ONLY sustain & vibrato instrument in the world that can play more than two notes at the same time. Hence, melodica has an unfair advantage over all instruments as you can play one man band like piano, AND obtain the expressiveness like violins or saxophones AT THE SAME TIME. Next time we would talk more about how to apply vibrato on melodica, it can take years of training to get a nice vibrato on melodica.
Anyway, here is me playing a slow song using a melodica with vibrato, you can hear how expressive the tone can be when you apply vibrato. The song is the main theme song of "Dragon Ball GT". I don't know if you guys know it or not, this theme song is actually a very romantic love song if you know the lyrics.