v-filter

app.vue

//https://jsfiddle.net/z57f9yLh/
//@author: https://jsfiddle.net/user/airyman/fiddles/

<template>
  <div id="app">
    <input type="text" v-filter="'[a-zA-Z]'" v-model="message">
  </div>
</template>

<script>
Vue.directive("filter", {
	bind: function(el, binding) {
  	this.inputHandler = function(e) {
    	var ch = String.fromCharCode(e.which);
      var re = new RegExp(binding.value);
    	if (!ch.match(re)) {
      	e.preventDefault();
      }
    };
  	el.addEventListener("keypress", this.inputHandler);
  },
  unbind: function(el) {
  	el.removeEventListener("keypress", this.inputHandler);
  },
  inputHandler: null
});

new Vue({
	el: "#app",
  data: {
  	message: "Ivan"
  }
})
</script>