The terms of the companion voucher plainly state that the tickets must be booked from the V, M, L, K or W buckets, meaning that the best use for it is when you can find a fare in any of those buckets that is less than double the lowest available fare for when you want to fly.

For example, if you wanted to buy 2 tickets at $250 each, but there was a $400 V fare available, it might be a good idea to use the voucher. If the V/M/L/K/W buckets are all more than twice the lowest fare, then it's not a good use.

A search for the words "Citibank companion" should bring up numerous threads discussing the use of these vouchers.

Mike