Add the -15% discount with your buy and then contact support with your 30euro voucher and they will refund the 30 euros when your order is dispatched. At least thats what happened when I was in a similar situation.
I'm sure I got a code when I had a voucher and that allowed me to combine vouchers online in one go. Maybe that was a voucher from a refund, but seems odd that you have not got a code from purchasing a voucher directly.
Add the -15% discount with your buy and then contact support with your 30euro voucher and they will refund the 30 euros when your order is dispatched. At least thats what happened when I was in a similar situation.