UPDATE:
1. I tried Facebook Messenger: Vueling don't accept Facebook Messages and ask that you instead use Twitter.
2. I created a Twitter account and sent a private message to @vueling. Within 1 hour they had updated the booking so that I was finally able to use the online system to claim EU compensation for the cancelled flight. Progress!
3. 11 days later Vueling denied the claim "your flight was cancelled due to air traffic congestion...extraordinary circumstance...not entitled to any form of compensation"

From the previous advice, it looks like the next step is MCOL or AESA in Spain - is that right? or is there a chance that "air traffic congestion" is a plausible extraordinary circumstance?