data: () => ({
  handle: null,
  ...
}),
mounted() {
  const mql = matchMedia('(max-width: 768px)');
  const onChange = () => this.handle = mql.matches ? '.image' : null;
  onChange();
  mql.addEventListener('change', onChange);
  this.$on('hook:beforeDestroy', () => mql.removeEventListener('change', onChange));
},<draggable
  :options="{ handle, ... }"
  ...
>