Contracting “des” into “de” in “je reviens de vacances”

Je reviens de XXX

This is actually a very specific sentence that could translate to “I’m coming back from” : you can’t really use another sentence to express the same.

You can use des vacances but when you are talking about a specific vacation:

Je reviens des vacances de février.

If you don’t, vacances is a generic, indefinite noun and doesn’t take an article:

Je reviens de vacances.

like we say:

Je rentre de vacances.
Je rentre de voyage.
J’ai besoin de vacances.
J’ai besoin de sucre.
J’ai besoin de sommeil.

There is a difference between :

Je reviens des grandes vacances. (I’m back from summer holidays)

and

Je reviens de grandes vacances. (I’m back from long holidays)

This is actually a case of haplology (repeating sounds avoidance). The undefinite article (des or du) disappears when it follows the preposition de

What is intended to say is:

Je reviens “de des” vacances.
J’ai besoin “de du” sucre.

La mort is not generic, it’s not de de la mort: I’m back from (some) death, but de la mort: I’m back from (the) death, so you say:

Je reviens de la mort.
Je reviens de la prison.: I’m returning from the jailhouse.

Other examples with revenir

Je reviens d‘examens. (de des examens)
Je reviens de prison. (de de la prison): I’m back from jail.