Rendezvous is French. It comes from rendez-vous, an instruction that means roughly present yourselves, or go to a place. In French the z of rendez and the s of vous are not said, and English kept that.
English changed other parts. French has a nasal vowel at the start; English speakers say a clear RON or RAHN with a real n. And where French gives weight to the end, English stresses the first syllable.
The plural is spelled the same, but you add a z sound: RON-day-vooz. Oxford lists this, and it also lists a lighter middle syllable, RON-dih-voo. Rendezvous works as a verb too: the ships will rendezvous at noon.