The naming on the voucher suggests it is good for 6 x VCTA exams, whether that's you using it 6 times or 6 different people using it once each (which is likely to be their intention)
Are you sure it was a VCTA you intended to achieve, or are you working towards your VCP? They may have issued you the wrong voucher.
I would ask Stanly if you want to be sure - I don't know the process or format of how VMware issues vouchers to VITAs.