When Vueling expects to cancel a flight due to low loads, the flight is available for booking right up until the cancellation decision. These cancellations are done in batches around the beginning of the month - well outside the deadline for EC261 compensation. A one-off March flight would never be cancelled this far in advance - it would be cancelled in early February.

When a flight is cancelled, Vueling automatically rebooks you onto the best alternative. If you don't like that flight, you can request a refund or change dates online.

Anyhow, if your March flight isn't available for purchase, it's probably sold out. But without telling us the route, there's only so much we can speculate about...