Hi,
I have a string instrument in Kontakt and I want to add vibrato to its sound, but I could not find any good tutorial on how to do this from Google and YouTube… If anyone could give me a walkthrough or any tutorial link for this I would really appreciate it.
The Kontakt instrument may, or may not not use samples that include vibrato. Either way it is instrument specific. What is the Instrument you want to use?
What Raino said. If the instrument allows it, you might be able to map a midi cc to control the vibrato with a mod wheel or fader, or just draw it in. But mod wheel/fader is cool, as you can play it with “feeling”, and add it while playing the part 
Sorry for my late reply… Both the the MVibrato and MIDI mapping works for me. Thanks very much for your help!
The instrument I’m using is this:
https://www.dropbox.com/sh/pidrs63sflej0yj/AAAJwrjw-GFraGDlZlHnkJbya?dl=0
But I have a problem. The sound of the instrument starts about 0.1 seconds late and it gets gradually louder. But I want the sound to start from the loud level and keep the loudness relatively constant without being gradually louder. This is because I need to play a fast melody with this instrument. I looked up all configurations for this instrument in Kontakt, but could not figure out how to make its loudness constant… I would appreciate anybody’s help.
Well that is going to be entirely dependent on the developer’s implementation. But I’d look for something named ‘attack’ and shorten it. Outside of the VSTi you could use a compressor to even out the sound somewhat, but that won’t help much with the slow attack. Sometimes a sound won’t/can’t work for a specific passage and you need to find something else that does - there is a reason tubas don’t play fast parts.