Is it possible to vassalise a nation that already owns a vassal

It is in diplomacy tab on your own state tab.

Excommunication seems to be a somewhat broken mechanic at the moment.

In fact you can only excommunicate another nation if their papal influence (the little number you see on the papacy icon) is lower than 5.

However, this is very unlikely to happen often, because gaining Papal influence is easy for most catholic nations and happens automatically.

The only way a country can drop below this boundary is if it just spent all its influence on a cardinal. So you can only get excommunicated if you raise your influence with the cardinals.

This does not make a lot of sense. As a comparison: In EU3 a country could get excommunicated when the relations with the Papal States dropped below 100.

So if you are really eager to excommunicate someone in EU4, keep an eye on the papacy tab. If you see that someone just raised their influence on a cardinal, you might have a chance to excommunicate them in the same month.