... why isn't there a vacation mode that says from this date to that date supress all notifications for this account???
Maybe because different people have different ideas and preferences of what a "vacation mode" might mean. Some don't want to receive notifications, others - don't want to have sync at all, someone else - to receive messages only to the family account, etc.
And how should the app know which account is
"this" and which is
"that"?
So, it would be hard to make a "one-size-fits-all" vacation mode. Rather than making additional configurations of what a vacation mode would mean, - you have the flexibility of switching off exactly what you need, -- notifications in this case, - as described in detail by @mikeone.
That's my (user's) perspective; developer's perspective on this could be different.