You might wanna listen to gospel artist and/or choirs that you like, and pay attention to the speed of their vibratos.
You have to do like what vocal trainers always talk about, which is:
"sign from your diaphragm"...which is done when you add volume. That way you're not singing merely from your throat or nose or whatever. But the notes are coming from a bigger place (the diaphragm).
It's like if you were in a place where you had to be quiet, like a library, then you wanted to get the attention of the person directly in front of you, with his back turned towards you - you softly say "hey?"
Now, what imagine if you wanted to get the attention of someone, let's say - across the street (not clear down the football field! lol...just across the street), then you'd say "HEYYY!"
Notice what you had to do with your voice? That came from the diaphragm! Much different than in the library! That's the beginning, which means: "projecting".
The second part is, when you know how to project, then you can pay attention to vibrato speed. If you hear your favorites use it slow, then slow yours up while projecting. If you like it fast, either way is up to you.
But lastly, if you listen closely, you might be able to tell that the strong slow vibrato is actually "dipping" in pitch, and not just volume! But the pitch dip is so slight, that you can't tell - but ends up sounding like (volume up, down, up, down) as it vibrates.
I would say the "down part" is where the pitch relaxes every so slightly and goes down for a micro second, and on the "up part", the pitch recovers again. So when you go all out and execute it, it sounds more like volume fluctuating, when in fact, there's a slight pitch fluctuating as well.
But you have to "observe" different singers & choirs, and might pick up on that.
The slow vibrato is very rich, in that, sometimes the "slowness" of it, brings about tension! You can slow it up so much to make people ALMOST cringe, but you don't take it that far, and suddenly you've created "tension", which translates into: "people feeling it".
In the end, you have to use your ears, and observe others who are doing what you are shooting for.
(By the way, i'm not a singer or anything, i'm just using a lifetime of "close-up observation")