Cannabis is federally illegal in the US, and the federation has its own enforcement teams that can come and get you even if your state's enforcement teams won't.
This is half true. Medical cannabis is now schedule III, with state programs explicitly placed into sched III (even without FDA approval) making it fully federal legal in that case.
Economic and monetary union is as a group of policies aimed at CONVERGING the economies. From your link.
The European Union consists of 27 countries.
25% of them did not adopt Euro as the currency.
"common" language is orthogonal here - it would be valid if you could legally use euro everywhere. You can't, it's not a currency in the quarter of the states. Sure, someone may accept it and offer you the exchange to the local currency.
Vatican City example is also not very good (to put it mildly), because Catholicism is a state religion. You're not going to be deported for being Sikh, yes, but it's akin to the Romanian not being deported form Portugal for carrying lei in his pocket.
Euro is NOT a common currency in the EU. It is by far the most popular. It is a common currency in the Eurozone countries. And these two are distinct from Europe as well.
I'd suggest you discuss your ideas with someone before posting them again.
You were ridiculed because I never stated the EU was a country. I said I didn't know how things work in the EU, not that everywhere you go in the EU it would be the same (in fact, I explicitly stated, I did not know how the system worked in Spain specifically to denote national differences in law in the EU). Your malicious use of feigned lack of reading comprehension merits the response.