Toolbar

List all configure toolbar

config.toolbar = [
  { name: 'document', items: [ 'Source', '-', 'Save', 'NewPage', 'Preview', 'Print', '-', 'Templates' ] },
  { name: 'clipboard', items: [ 'Cut', 'Copy', 'Paste', 'PasteText', 'PasteFromWord', '-', 'Undo', 'Redo' ] },
  { name: 'editing', items: [ 'Find', 'Replace', '-', 'SelectAll', '-', 'Scayt' ] },
  { name: 'forms', items: [ 'Form', 'Checkbox', 'Radio', 'TextField', 'Textarea', 'Select', 'Button', 'ImageButton', 'HiddenField' ] },
  '/',
  { name: 'basicstyles', items: [ 'Bold', 'Italic', 'Underline', 'Strike', 'Subscript', 'Superscript', '-', 'CopyFormatting', 'RemoveFormat' ] },
  { name: 'paragraph', items: [ 'NumberedList', 'BulletedList', '-', 'Outdent', 'Indent', '-', 'Blockquote', 'CreateDiv', '-', 'JustifyLeft', 'JustifyCenter', 'JustifyRight', 'JustifyBlock', '-', 'BidiLtr', 'BidiRtl', 'Language' ] },
  { name: 'links', items: [ 'Link', 'Unlink', 'Anchor' ] },
  { name: 'insert', items: [ 'Image', 'Flash', 'Table', 'HorizontalRule', 'Smiley', 'SpecialChar', 'PageBreak', 'Iframe' ] },
  '/',
  { name: 'styles', items: [ 'Styles', 'Format', 'Font', 'FontSize' ] },
  { name: 'colors', items: [ 'TextColor', 'BGColor' ] },
  { name: 'tools', items: [ 'Maximize', 'ShowBlocks' ] },
  { name: 'about', items: [ 'About' ] }
]

How to add custom toolbar

First at all, you need need to upload content of plugin to website

ckeditor/plugins/custom_plugin

then you configure ckeditor with vue-ckeditor

<template>
  <div class="app">
    <ckeditor v-model="content" :config="config"></ckeditor>
  </div>
</template>

<script>
import Ckeditor from 'vue-ckeditor2'

export default {
  data () {
    return {
      content: '',
      config: {
        toolbar: [[ 'Bold', 'Italic', 'Underline', 'Strike', 'Subscript', 'Superscript', 'Custom Button' ]],
        height: 300,
        customConfig: '/ckeditor/custom_config.js',
        extraPlugins: 'custom_plugin'
      }
    }
  },
  components: { Ckeditor }
}
</script>