Vacation mode allows you to “freeze” your account so that other players cannot attack or interact with your account in your abscence, at the expense of not being able to progress during this time. Keep in mind the following:
No upgrades of any type can happen during vacation mode. If you have buldings or researches currently in progress, you won’t be able to go into vacation mode until you cancel everything first.
Your troops cannot be traveling while in vacation mode. If you have troops currently on their way, you will have to cancel all orders and wait for their return before enabling vacation mode.
Minimum vacation period is 48 hours, to prevent abuses. Once vacation mode is enabled, you will not be able to return to your account until 48 hours have passed (You can’t log in); it is not possible to enable and disable vacation mode in rapid succession.
Maximum vacation period is 15 days. If you do not return to your account before this time, vacation mode will automatically expire and other players will be able to interact with your account again.
Vacation mode will not save you from existing attacks. If you are currently being attacked, said attack(s) will proceed as usual even if you enable vacation mode. You will only be protected from future attacks, not currently on-going ones.