Get a good shoulder rest that will stabilize your hold when doing vibrato. You will be suprised to realise how much easier it`ll be with a better shoulder rest.
Make sure your palm is away from the neck. Don`t stick it to the neck.So, lower your hand slightly and relax totally.
For more professional advice/information...you might wanna post in this professional website with lots of experienced violinists. Check out our orchestral recordings there,and you can upload your playing there also. The latest amazing recording was done by one of our advanced member on Paganini`s Caprice 5.
http://www.soloviolin.com
Thank You.