A question you might want resolve here:   is it a true vibrato, or is it an uncontrolled wobbling? The latter usually means a lack of good breath support, or possibly a bit of uncertainty as to the precise pitch you're trying to sing.

A normal vibrato, which most of the time and in most styles of singing, is desirable and is something that most people don't even notice, either in their own voices or in others—unless, of course, someone makes a fetish of it, and then it can become an obsession.

Just for kicks, I have just spent a most interesting hour bouncing all over YouTube, listening to opera singers such as soprano Anna Netrebko (!! She ain't no "fat lady," she's gorgeous!!), baritones Dmitri Hvorostovsky and Bryn Terfel (couple of pretty handsome dudes), and folk singers Ewan MacColl, Mary Black, Jean Ritchie, Pete Seeger, and a whole bunch of others. And they all exhibited vibrato in their singing! Surprise, surprise!

The idea that folk singers don't use vibrato is simply not true. What may mask this to many peoples' ears is that in short notes, vibrato doesn't have time to establish itself. But if a singer—folk or opera—sings a long, sustained note, it is most definitely there. What may make it more noticeable in the voices of opera singers is that the music they sing tends to have many more long, sustained notes.

Go to YouTube and listen for yourself. You'll hear it.

Don Firth