Вы все делаете неправильно. Вот рабочий код для концепции:
HTML:
<code><div id="app">
<multiselect
v-model="value"
:options="options"
:multiple="true"
track-by="library"
:custom-label="customLabel"
>
</multiselect>
<pre>{{ value }}
Javascript:
new Vue({
components: {
Multiselect: window.VueMultiselect.default
},
data: {
value: { language: 'JavaScript', library: 'Vue-Multiselect' },
options: [
{ language: 'JavaScript', library: 'Vue.js' },
{ language: 'JavaScript', library: 'Vue-Multiselect' },
{ language: 'JavaScript', library: 'Vuelidate' }
]
},
methods: {
customLabel (option) {
return `${option.library} - ${option.language}`
}
}
}).$mount('#app')
Jsfiddle