How can I use multiple condition in v-if in VueJS?

Maybe it’s the way you are treating empty strings as false-ly values and || is saying: show fullname if any of the two (left/right) expressions are true, which is not what you want I think.

Try this instead:

 <li v-for="item in Names" v-if="item.FullName !== null && item.FullName !== ''"> 

Also, judging from your code, {{ FullName }} should be {{ item.FullName }}