It's entirely Broadband Choice's decision to give out the vouchers. Virgin and other companies have absolutely zero things to do with the vouchers or bonuses for signing up, you wouldn't expect Tesco to honour a deal you seen for Asda, so why would Virgin pay out the bonus of another company?
Your only option is to contact Broadband Choice and go through the hoops with them.
** I work for VirginMedia but all opinions posted here are my own.