"Non-Refundable" F (booked into C or D fare classes) is pretty much a misnomer, particularly for anyone who flies VX with any regularity. In the event of a cancel/change, there's no fee and the entire fare goes back to your Travel Bank. Then, use the travel bank for future travel. I probably have $200 - $1000 in my travel bank at any given time, simply as the result of frequent changes and cancellations of F fares.

One other thing: the fully refundable F fares (booked in to J class) are transferrable, meaning you can change the name on the ticket. This is rarely useful, but in cases where VX runs a 10%/20%/X% discount on all F fares, booking a fully refundable F ticket becomes much cheaper and you can transfer it to you travel companion later. Good for impulse trip bookings, imo.

Note: this distinction, i.e., "non-refundable" meaning no change/cancel fee w/ full refund does NOT apply do Non-Refundable MCS.