The terms of the Vueling flight credit program are remarkably flexible with the exception of the date of expiry, as you noted in your original post.
The conditions of TimeFlex tickets are included in Vueling's Conditions (Contract) of Carriage under Section 9.
Interestingly, the Conditions of Carriage do not make reference to free cancellation (for travel credit), though Vueling's fare comparison chart does reference this. Since the cancellation would be in the form of "Flight Credit," it would appear to be subject to the terms of the Flight Credit program (linked above) and to have the same restrictions as your current credit (i.e., very few restrictions).
The one caveat is that I cannot find anything on the website that gives a specific stated amount of time the credit would be good for when canceling a TimeFlex fare. It's probably worth calling/texting/Tweeting Vueling to get a little more information.
One other option: Follow the Christopher Elliott strategy for solving your own consumer problem. In your case, you could write a short letter/message to Vueling's Customer Experience Manager (her address and e-mail address are on the elliott.org website's list of contacts for Vueling).
Be positive and be concise. You want 4-6 sentences that state the problem (credit expiring but pregnant) and state your proposed solution. I think you are more likely to get an extension of the credit for a year than a refund, so that would be my ask. It's usually helpful to throw the company a bone like "We've really enjoyed our past Vueling experience and are looking forward to flying again" (if you've flown before) OR "We've heard so many great things about Vueling and are really looking forward to the chance to fly on your airline" (if you haven't flown Vueling in the past). AVOID THREATS like "we will never fly Vueling again if our credit is not extended" -- that greatly decreases your likelihood of success.
A positive, concise, clear message to the right person often results in success that cannot be obtained from a customer service rep who is just following what the computer screen tells him is company policy.
Good luck!
