I use a diaphragm vibrato on flute (and its cousin the ocarina), but I can't make that work on euphonium. Jaw vibrato for me. I'm also not one for waggling every note longer than a fast eighth-note; I tend to use vibrato only on longer notes, and bring it in about halfway through the note, like a good pop singer might do it.
David Bjornstad
1923 Conn New Wonder 86I, Bach 6 1/2 AL
2018 Wessex EP100 Dolce, Denis Wick 4ABL
2013 Jinbao JBEP-1111L, Denis Wick 4AM
2015 Jinbao JBBR-1240, Denis Wick clone mouthpiece of unknown designation
Cullman (AL) Community Band (Euph Section Leader)
Brass Band of Huntsville (2nd Bari)