I am not sure how to do vibrato specifically on acoustic guitar, not electric. Any ideas? Thanks.
Vibrato on acoustic guitar

Cadorange,
Vibrato technique should be performed the same way regardless of guitar type. I think you might be having a problem because it's a bit more difficult to hear a vibrato on an acoustic guitar since it's not plugged into an amp.
Otherwise, I'm not too sure about a better way to vibrato on acoustic!

There are a lot of different kinds of vibrato, but all should be executed on both guitars. Electric and accustic.
You should check out this thread on the forum:
https://www.guitartricks.com/blog/10-Things-You-Need-to-Know-for-Killer-Vibrato
The one or the other different vibrato might be a bit more difficult to play on the accustic (like the "bend-kind of vibrato" but it is a nice "workout" for your fingers and you will get a lot better at your electric guitar vibrato after practising for a while on the accustic.

actually When I make a vibrato on my acoustic guitar, I can't hear the difference! So, it is my skill's fault or my ear's fault?
When it comes to vibrato an old classical guitar teacher told me that it's way better to move your finger up and down vertically instead of horizontally. I hope that helps!

A lot of technique comes down to hand & finger strength when playing steel string acoustic guitar. Keep some focus on your posture and breathing to make performing techniques such as vibrato easier