I use my ring finger, but with the middle finger sort of pushed up against it to brace it.
Its not easy to get right, but honestly, stick with it, and it'll come good.
One bit of advice i'd give is make sure you aren't gripping the neck with your thumb too hard.
This makes it harder to slide between the notes fast or smooth enough.
As an excercise, set up a metronome, and practice the vibrato on maybe the 4th beat (if assuming 4/4), but do it without gripping the neck at all.
This will make it easy to slide quickly.
Once this is ingrained in you, slowly start using your thumb again, until you build up to being able to do it without a struggle.