Vibrato is an effect used often to add expression to music of all kinds, and certainly for classical violin it's an essential technique to work on. 

Here are some things to keep in mind when using vibrato:

Keep your vibrato finger flexible. If the finger is rigid, there is no kind of vibrato that's going to be satisfying. It's very important to have flexibility with the finger joint that contacts the string.

Vibrato only decorates a note from below, so it should never rise above a note. The human ear tends to hear a vibrated note as the highest pitch that is reached, so don't go above the note you're playing.

Beware of the "dead note" - this is when you have singing lines that have a sudden note without vibrato. This is one of the worst habits you can develop in string playing. Be on guard for dead notes in your playing. 

To have continuous vibrato, you must be able to have one finger hand off to the next as you're playing so that you won't lose the vibrato.

Another thing to work on is varying the speed of the vibrato while playing. All the great soloists have different speeds for their vibrato. For example a wide vibrato will sound best when playing a romantic concerto, especially if it's slow, but if the piece increases tempo and intensity the vibrato may get more narrow.

Metronomes are very useful for getting out of your "default vibrato" comfort zone. Try doing vibrato exercises at 60 BPM as a way to count your "vibrato wiggles" and increase your vibrato speed gradually over intervals.

It's important not to have tension in your fingers when you're practicing vibrato. If you feel tension building up, take a rest and practice it again some other time. Practicing vibrato with a metronome will help you get comfortable working with different vibrato speeds at the different intervals.

Practice along with a metronome and playing division of 2, 3, 4, 5, 6, and 8. Once you go through those divisions, increase the speed on the metronome to challenge yourself to practice faster vibrato.

Another tip for practicing vibrato: find a recording of your favorite soloist, slow it down if you have to, and figure out their vibrato speed. 

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