I have noticed that, at least in recent versions of Vuetify, you need to specify both $body-font-family and $heading-font-family to change the fonts of everything from Roboto to something else in your overrides (following these instructions). The redefinition of $headings seems to be necessary since it is defined in _variables.styl and depends on $heading-font-family. Note that Vuetify will be moving to SCSS in 2.0 so there will be a new process at that point.
$body-font-family = 'Barlow Condensed', sans-serif
$heading-font-family = 'Barlow Condensed', sans-serif
$headings = {
  h1: { size: 112px, weight: 300, line-height: 1, letter-spacing: -.04em, font-family: $heading-font-family },
  h2: { size: 56px, weight: 400, line-height: 1.35, letter-spacing: -.02em, font-family: $heading-font-family },
  h3: { size: 45px, weight: 400, line-height: 48px, letter-spacing: normal, font-family: $heading-font-family },
  h4: { size: 34px, weight: 400, line-height: 40px, letter-spacing: normal, font-family: $heading-font-family },
  h5: { size: 24px, weight: 400, line-height: 32px, letter-spacing: normal, font-family: $heading-font-family },
  h6: { size: 20px, weight: 500, line-height: 1, letter-spacing: .02em, font-family: $heading-font-family },
  subheading: { size: 16px, weight: 400 },
  body-2: { size: 14px, weight: 500 },
  body-1: { size: 14px, weight: 400 },
  caption: { size: 12px, weight: 400 },
  button: { size: 14px, weight: 500 }
}