set the text of the buttons in your array
vex.dialog.open({
message: 'Are you absolutely sure you want to destroy the alien planet?',
overlayClosesOnClick: false, // set false to click out
callback: function (value) {
console.log(value);
},
buttons: [
$.extend({}, vex.dialog.buttons.YES, { text: 'Your Button For Yesy' }),
$.extend({}, vex.dialog.buttons.NO, { text: 'Your Button For No' })
]
});