How do I let people know that I'll be on vacation?

Use the Unix vacation program. The vacation program will only send an auto-reply to each person who sends you mail only once, so it's best to include your return date. Also, vacation will not send auto-replies to mailing lists (it only replies to messages that have your address in the To: field)
Open up a terminal window and type:

How do I setup my account to forward mail to another location, but keep a copy of the message here too?

Mail forwarding is handled by the file .forward in your home directory. To start forwarding your mail, create this file, using any text editor and insert entries according to the following examples.

Assume your username at the IAS is ezeno, then to forward to your account and keep a local copy, you would use: