<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by RustyC:
...look into the 2 or 3-day tours offered by leading guesthouses in Saigon or Hanoi. They're so cheap that you'd be hard-pressed to go someplace like the Mekong Delta or Ha Long Bay on your own for cheaper. </font>
Results vary on those tours... could be great, but you risk being packed in with 30-40 people just to save a few dollars.
A better bet is to get yourself to Cat Ba City (for example by train/hydrofoil), then set up a private/small group tour with a local operator at a hotel there. Rented a boat that could seat 16 for all of $25 for the day including lunch... wife and I had lots of space, no hassles.
Sure, it was a few dollars more, but it was worth it -- everything's cheap to begin with! Total came to under $100 total for two, for three nights, including all transportation, food, lodging, etc.
But whatever you do, don't miss Ha Long Bay! Enjoy.