The vacation mode of WhatsApp is essentially the possibility to disable all group notifications so that we're not disturbed. This function directly affects archived chats, those that we hide from the main conversation window, and return to the beginning as soon as we receive a new message. Well, with vacation mode, they do not return to the main window, even if messages come in.
With the vacation mode, chats are hidden until the user decides to reactivate them. The purpose of the vacation mode is to let users take a real break from the conversations of certain groups; something more efficient than silencing the cell phone or group notifications. This mode is based on the silent mode, already available on Android.
This feature is now available in WhatsApp betas for Android, via the path Settings > Notifications > Ignore archived chats. However, it has not yet been definitively implemented in the final versions available to all users.
The fact is that for now, everyone is wondering where, when, and how vacation mode will come to Android (we assume it will at some point), but it seems that its arrival will be one of the most welcomed developments in the (slow but steady) evolution of WhatsApp Messenger for Android.
It is worth remembering that direct competitors such as Telegram (or, for that matter, MODs such as GBWhatsApp, which the official version does not like very much) are well ahead in terms of the evolution of the app now owned by Facebook.