EnderH

mounted () {
  window.addEventListener('scroll', this.handleScroll)
}

methods:  {
  handleScroll () {
    var scrollTop = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop
    console.log(scrollTop)
  }
}

handleScroll () {
  var scrollTop = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop
  var offsetTop = document.getElementById('bar').offsetTop
  if (scrollTop > offsetTop) {
    this.barFixed = true
  } else {
    this.barFixed = false
  }
}

.isFixed{
   position:fixed;
   background-color:#Fff;
   top:0;
   z-index:999;
}

<div :class="{isFixed: barFixed}"></div>

destroyed () {
  window.removeEventListener('scroll', this.handleScroll)
}