Apenas para documentar a solução proposta no grupo 'Help':
That is due to the browser, if it does not limit the form validation it will submit anyway.
You can use @submit.prevent="methodName" in form tag and before submitting will run the methodName where you can check anything.
Keep in mind that Vue does not have a validator itself but you can add one. As form validator I like https://github.com/fergaldoyle/vue-form