[Vuejs]-Render custom header in element-ui to show bootstrap tooltip?

The id needs to be defined inside the attrs object:

renderHeader(h, { column }) {
  return h(
    'div',
    {
      attrs: {
        id: `tooltip-target__${column.property}`
      }
      class: 'some-class'
    },
    [
      column.label,
      h(
        'b-tooltip',
        {
          attrs: {
            triggers: 'hover',
            target: `tooltip-target__${column.property}`
          }
        },
        column.label
      )
    ]
  );
},

I found a good example in the documentation:
https://v2.vuejs.org/v2/guide/render-function.html#The-Data-Object-In-Depth