Whats wrong paman

html

<a href='javascript:void(0);' v-on:click='goRegister.redirect(checkSession("hashvirtual_ref"))'><strong>Member</strong> Register</a>

js

var goRegister = new Vue({
    methods: {
        redirect: function (username){
            if (username != ''){
                window.location = 'https://dashboard.hashvirtual.cloud/p/register.html?ref=' + checkSession("hashvirtual_ref");
            } else {
                window.location = 'https://dashboard.hashvirtual.cloud/p/register.html';
            }
        }
    }
})

#JS

var goRegister = new Vue({
   el: '#app',
    methods: {
        redirect: function (username){
            if (username != ''){
                window.location = 'https://dashboard.hashvirtual.cloud/p/register.html?ref=' + checkSession("hashvirtual_ref");
            } else {
                window.location = 'https://dashboard.hashvirtual.cloud/p/register.html';
            }
        }
    }
})

#HTML

<div id="app">
  <button v-on:click="redirect">Redirect</button>
</div>

Note

Vuejs itu bekerja dengan element id atau class. tengok lagi user guidenya. jangan malas baca.