Learning vibrato is difficult only if you have improper fundamentals in the left hand. If you are moving improperly towards notes, or pressing down too hard on the fingerboard, you will find vibrato will complicate intonation and playing fast. On the other hand, if you really work hard on your left-hand technique, you will find it a lot easier to learn vibrato. Here are some things that must be in place to do vibrato properly.
1. Nice relaxed hand – If you have tension in your hand (really anywhere), it is going to cause some problems getting a nice fluid motion. Try relaxing as much as possible from your elbow all the way to your fingertips.
2. Finger angles back – If you reach towards notes, your fingers are changing angles in accordance with the fingerboard. Basically, that means you are putting your hand out of place to create proper vibrato motion. Place fingers on the very tip of the string, and stay away from reaching towards notes.
3. Finger placement – Place your fingers on the fingerboard as close as you can to your nail. If you use more the meat of the finger instead, you are going to find it difficult to have limited pressure on the fingerboard to keep your hand relaxed. Really, point one that I made above and this point go hand in hand. To get a relaxed hand you have to just barely use enough pressure on the fingerboard to get the pressure you need to make the note sound. Over pressing will make you force the vibrato which will make it harder.
I see some students have proper fundamentals for being able to do vibrato well in 4 weeks, while others it takes 5+ years. Just focus on left-hand fundamentals and you should be fine. Since shifting and vibrato is similar in the proper technique you need to do it, I highly recommend watching this violin shifting video for more tips on left hand.