Hello 
I love to sing, but problem is, when I sing, there's hardly any vibrato. Though I tried a lot of practices like, lowering my larynx(?), singing louder, widening my throat. But doesn't work.
Does the vibrato occurs in the stomach or throat ?
And how do you differentiate between a forced vibrato and a natural one?
Thanks ! 
Well i actually grew from a forced vibrato, to a natural one.
I just kept on singing, with my forced vibrato and it turned into a beautiful natural one.
Your question is does vibratio occur in the stomach or throat? Neither.
It happens in your diaphram. Okay well to show you a little how vibrato should sound like.
Find your ribs, and you know how they go down like an upside down V. In the space, while just holding a tone, press it repeatedly and you will get an impression of what vibrato is.
You know how an ambulance goes weeee wooo, weee wooo, try and do that with a tone. like Ahhhhhhh Ahhhh,
Listen to people who sing with vibrato and you will learn when to use it, and when not to use it.
I really started to do it forced, and slow, then as i started singing along with songs it started to come naturally. And you don't sing with your throat.
You sing with your diaphram. Which gives you alot of strength, when you sing try and get strength from below, with your lungs, ect and you will be fine
Even watch some youtube videos for help
Y/W