My P31-2 came with a rigid vang. It gives you one more way to shape your sail for maximum efficiency. The rigid vang is handy because it eliminates the need for a topping lift to keep the boom up.
You might try sailing for a year without a vang and see if you think it would have helped you get a few tenths of a knot. Depending on wind conditions, condition of your sail, and point of sail, the usefulness of the vang varies a bunch. If, after a year, you think that life would be so much better with a vang, you can figure out if the additional cost is worth the extra, incremental, sail control.