It's not because English is a vernacular language for most people that it is the de facto lingua franca for the rest of the world. Let's not forget that for a very long time, French was the language of diplomacy for a few centuries, and the official language in the European Union, until the UK and Ireland joined in and bullied their way through.
That's b/s. The European Communities did never have a single official language; it used all of the Member States' languages in parallel from the beginning. (Well, some institutions do have a working language, eg the European Court of Justice uses French internally, probably because its located in Luxembourg.)
BTW, when the UK and Ireland joined, there was no European Union.
Please don't confuse lingua franca and a vernacular language.
Please don't confuse modern French and lingua franca, which originally referred to the Frankish language, a West Germanic language only remotely related to Romance languages such as French.
In other words: The rest of the world speaks English because: a/ it's an easy language, b/ most of English speakers are too lazy, or can't be bothered to learn another language.
That's only because English is already useful enough, so there is no need to learn a different language.
The reason for English being such prevalent is, of course, the British Empire spreading it.