First of all, there's no "compensation" due. Weather is an extraordinary circumstance and you have no chance of convincing anybody that a pilot / ATC decision to divert to BCN is anything other than extraordinary. It won't matter that other planes managed to land or that EAS and SDR are closer.

Vueling does, of course, have a "duty of care" requirement. I have no idea whether an 8-hour coach ride qualifies, and it does make no sense at all that VY would send a coach to BIO instead of rebooking passengers on VY flights.

You need to stop using the word "compensation" and simply ask for "reimbursement of expenses". (hotel, meals and replacement flight) If VY reject that again, you will have to take them to MCOL.

If you keep using the word "compensation" you will continue to give VY an excuse to keep denying your claim.