vuejs set a radio button checked if statement is true

You could bind the checked attribute like this:

<div v-for="portal in portals">
  <input type="radio"
         id="{{portal.id}}"
         name="portalSelect"
         v-bind:value="{id: portal.id, name: portal.name}"
         v-model="newPortalSelect"
         v-on:change="showSellers"
         :checked="portal.id == currentPortalId">

  <label for="{{portal.id}}">{{portal.name}}</label>
</div>

Simple example: https://jsfiddle.net/b4k6tpj9/