Given that the eMac is CRT-based, it'll probably been using a bit of phantom power (~5 watts maybe?) if it's still plugged in. There's no harm to leaving it unplugged.
As for the MacBook, how long is your vacation? Apple recommends cycling the battery at least once per month. If you're going to leave it alone for longer than that, they recommend that you drain the battery to 50% and take it out of the MacBook.