How identify a long running transaction

Using 8.4.1. How can I

1) Get the transaction id of the oldest serializable transaction and the 
next transaction?

2) A  list of all active transactions

I have searched the docs and tried Google but I must not be using the 
right terminology.

Also,  am I correct in assuming that  vacuuming is not blocked by a long 
running read committed transaction but only by the oldest serializable 
transaction?

Any references to relevant documentation would be appreciated.

Bill