Here is one way.
<span v-for="(val,key,index) of person"> key: {{key}}, val: {{val}}, index: {{index}} <span v-if="index != Object.keys(person).length - 1">, </span></span>
Works Great
<div id="app"> <div v-for="(item, index) in items"> <div v-if="index == items.length - 1">yes</div> {{ item }}, {{ index }} </div></div>
You can also "cheat" by inserting the comma before each item, as it's easier to check for the first item (key !== 0).
key !== 0
<span v-for="(val, key) in person"> <span v-if="key !== 0">, </span> {{key}}: {{val}}</span>