When playing single lines, my habit is to vibrate a string up and down on the uke-- comes from electric guitar playing, I guess. But it don't feel right with the nylon strings.
How do you experienced ukulistas vibrate: up and down or side to side?
I use up-and-down vibrato on ukulele or on guitar. I've never quite gotten the hang of the back and forth vibrato technique, but I can do it on occasion. Just not reliably.