data() {
 return {
   username: '',
    password: '',
  }
}

<el-input v-model="username" type="text" name="text" placeholder="账号" autocomplete="off"><i slot="prefix" class="el-input_icon el-icon-user"></i></el-input>

<el-input v-model="password" type="password" name="pwd" id="pwd" placeholder="密码" autocomplete="new-password"></el-input>

<el-input v-model="username" type="text" name="text" placeholder="账号" autocomplete="off"><i slot="prefix" class="el-input_icon el-icon-user"></i></el-input>

<el-input v-model="pwdCover" type="password" name="pwd" id="pwd" placeholder="密码" autocomplete="new-password"@input="setPassword"></el-input>

script
data() {
 return {
   username: '',
    password: '',
    pwdCover: '',
  }
},
method: {
 login() {
   this.pwdCover = this.pwdCover.replace(/\S/g, '●');
    // 登录请求，失败时恢复pwdCover
    this.pwdCover = this.password;
  },
  setPassword(val) {
   this.password = val;
  }
}

<el-form-item>
 <el-input v-model="username" type="text" name="text" placeholder="账号" autocomplete="off"><i slot="prefix" class="el-input_icon el-icon-user"></i></el-input>
</el-form-item>
<el-form-item>
 <el-input v-model="pwdCover" type="text" name="pwd" id="pwd" placeholder="密码" autocomplete="off" @input="setPassword"><i slot="prefix" class="el-icon-lock"></i></el-input>
</el-form-item>

setPassword(val) {
  let reg = /[0-9a-zA-Z]/g; // 只允许输入字母和数字
  let nDot = /[^●]/g; // 非圆点字符
  let index = -1; // 新输入的字符位置
  let lastChar = void 0; // 新输入的字符
  let realArr = this.password.split(''); // 真实密码数组
  let coverArr = val.split(''); // 文本框显示密码数组
  let coverLen = val.length; // 文本框字符串长度
  let realLen = this.password.length; // 真实密码长度
  // 找到新输入的字符及位置
  coverArr.forEach((el, idx) => {
    if(nDot.test(el)) {
      index = idx;
      lastChar = el;
    }
  });
  // 判断输入的字符是否符合规范，不符合的话去掉该字符
  if(lastChar && !reg.test(lastChar)) {
    coverArr.splice(index, 1);
    this.pwdCover = coverArr.join('');
    return;
  }
  if (realLen < coverLen) {
    // 新增字符
    realArr.splice(index, 0, lastChar);
  } else if (coverLen <= realLen && index !== -1) {
    // 替换字符（选取一个或多个字符直接替换）
    realArr.splice(index, realLen - (coverLen - 1), lastChar);
  } else {
    // 删除字符，因为 val 全是 ● ，没有办法匹配，不知道是从末尾还是中间删除的字符，删除了几个，不好对 password 处理，所以可以通过光标的位置和 val 的长度来判断
    let pos = document.getElementById('pwd').selectionEnd; // 获取光标位置
    realArr.splice(pos, realLen - coverLen);
  }
  // 将 pwdCover 替换成 ●
  this.pwdCover = val.replace(/\S/g, '●');
  this.password = realArr.join('');
},