This is something I have struggled with in the past as well...
Maybe there is a better solution to this, but I think you have to make each of the items a component, then they will have their own scope.
So in your v-for you have this for example:
<a v-for="contact in contacts">
<contact :contact="contact"></contact>
</a>
Then you have access to all of the same data inside your contact component,
"isSelected" will now have it's own scope if you define it inside the data object in your contact component.
Hope it helps!