How to return custom fields in GraphQL
For anything else than base element fields, you'll need to use an inline fragment to fetch your custom fields like so:

query test {
  entries(section: "vacancies", limit: 20) {
    title
    expiryDate
    ...on sectionHandle_entryTypeHandle_Entry {
      description
    }
  }
}


Replace sectionHandle and entryTypeHandle with your values, I guess something like vacancies_vacancies_Entry