[duckduckgoose]

They would have to get within range to do that, and there would be no point in doing that at all.
If a alien race was war like it would be a conquering civilization, they would try to take our planet from us for its natural resources and make us slaves, to do that they would have to get within range of our planet and our nuclear weapons could strike their ships. If they managed to land ground troops we would have a fighting chance against those as well, we know the terrain, were used to the planets weather patterns and its gravity. We have huge standing armies and our weapons would kill any organic creature just as well as they kill humans.

It also isn't exactly feesable to vaporize a planet, raze it with planetary bombardment yes. But to actually vaporize it? That would be extremely difficult indeed, and take unthinkable amounts of energy to do it.