If this is the same type of voucher I got for DBC last year than you can use it to buy any ticket at an LH office or at the airport. It doesn't affect the mileage earning ability of the ticket - it just substitutes cash. You just can't get a refund in cash for such a ticket and you don't get the difference paid out if the ticket is cheaper than the vouchers value.

Also you could cash the voucher in at half the value.

And you can use it with full value for buying tax free items on board your fligth.