I tried doing vibrato like how I've been doing vibrato on my violin and I found that it works. Meanwhile, I tried to do vibrato by bending the strings on guitar in a "up and down" motion, there was no vibrato at all. I tried to imitate professional guitar player by bending to get vibrato but it doesn't work. My friend told me "bending" only works on acoustic guitar, and for the best, on electric guitar, right? I'm a complete beginner in guitar playing and do not have much idea about the skills in guitar playing.
I am a little confused by the collection of descriptions and comparison of "imitating professional guitar player...".
When you say professional guitar player do you mean classical guitar player or electric guitar?
Also, the classical guitar is acoustic so what you friend told is very confusing. So did you mean steel string acoustic as opposed to nylon string (classical).
In my experience both techniques work on the classical. Both back and forth as well as up and down will produce an effect. I personally prefer back and forth (and use it on the electric too). If you are not hearing the effect there could be several reasons. The first is that classical guitar strings will not bend pitch very much for a small deflection but steel strings will. The amount of pitch change will be different for wound and unwound strings. It will also be a function of which tension and thickness of string you have on the classical.
As a counter example to the electric guitar and up-down vibrato, many jazz guitarists used flat wounds on the electric (similar to the feel of violin or upright bass strings) and the G string is usually wound. It is darn near impossible to bend the flat wound G string and get considerable amount of change in pitch.
Lastly, the acoustic guitar top naturally moves up and down (out of the natural rest plane), and slightly back and forth but not "side to side" (i.e. not in the plane in the direction of the side of the guitar). As a result any side to side movement does not pump or drive the bridge effectively. In that regard what you're calling up and down vibrato may not be as effective as side to side but it should still work. The side by side vibrato mechanically pumps the bridge and sustains the note. What is likely happening is that the up and down vibrato is affecting the pitch but the overall note is not being sustained. Practice will increase your ability to control these factors.
I know many classical guitarists who employ both techniques and have even heard classical guitarists string bend like a blues player (it's not as bold but works).
There are three different mechanisms at work in vibrato on the different instruments:
- On fretless instruments, you modulate the length of free string, i.e. basically by the same mechanism you also use to play different notes. Easy enough done by rolling the finger a little back and forth along the string (longitudinally). This technique is quite objectively the best one, but unfortunately it's not really available on fretted instruments – although I've seen it done; Tal Wilkenfeld sometimes uses a very wide longitudinal vibrato on fretted electric bass, actually moving her fingers over the frets, so it comes out as a kind of turn ornament.
- On classical guitar, you also use lateral motion, but the mechanism is actually very different: whenever moving your hand towards the head, you pull the string with you a bit and thus increase the tension of the sounding part, which raises the pitch (unlike on fretless, where moving towards the head lowers the pitch due to the longer string). Unlike on bowed strings, you should not roll the finger and hand around the arm axis because that doesn't change the tension, instead you should give the finger as much linear movement as possible.
This technique can be used in principle on all string instruments with a fingerboard, however it works by far best on nylonstrings because a) these have good friction on the finger without cutting in (so you can actually alter the tension significantly) b) they don't have as much tension by themselves as steel strings do, so even a small change in tension causes a significant pitch modulation.
- On steelstring guitar, you do this bending thing, i.e. a transversal motion along the fret rather than the string. Bending always raises the pitch (which is immediately a bit of a problem because classically speaking, vibrato should happen around or even below the target note, not strictly above it. Guitarists don't necessarily care about “correctness”, but as a matter of fact they often start from a whole tone lower, bend that up towards the target note, and only do the vibrato there).
The reason bending has a greater effect than lateral vibrato on steelstring guitars is that you don't depend on friction along the string, but can instead “hook” into it and thus increase the tension much more strongly. Even more importantly, you get a mechanical advantage by moving transversally (kind of a lever effect), which is why it's possible to bend up by three or even four semitones on an electric guitar – that would be unthinkable with lateral friction-tension.
The flip side of the mechanical advantage is that when the string stretches under the higher tension, the displacement is amplified and you need to move the finger quite a way along the fret even if it only stretches by fractions of a millimetre. This is not such a problem on steel strings because they don't stretch much to begin with (steel having a high Young's modulus). OTOH, nylon has a much lower Young's modulus, i.e. you can easily “bend” the string in the sense of stretching it out, but this doesn't actually change the tension very much and thus also has not a very impressive effect on the pitch. That's why Western/electric-guitar vibrato doesn't work as well on classical guitar as lateral vibrato does. But anyway, for vibrato per se I would argue that the longitudinal techniques are in every way preferrable; steelstring guitars don't allow this, but on the bonus side they support wider bending.
On nylon strung classical guitars, the commonly used vibrato is like that on a violin, from bridge to nut, within a single fret. It varies the pitch very slightly, and also the tone of the note.
The wider vibrato used on steel strung and electric guitars doesn't work too well. That vib actually changes the pitch more, by stretching the strings, laterally to their lie, which is fairly ineffective with nylon strings. incidentally, the 'classical' vib still works on steel strung guitars - it's just quite subtle in comparison to bending.
Which pro guitarist did you try to copy moving the string 'side by side'? Pro classical guitarists (and good amateurs!) use the violin style vib.