If you use a CS25 or CS50, for example, the total price of the ticket will reflect the deduction (e.g. ticket price $200, use a CS50, total ticket price in left hand corner states $150). The use of say, VDB vouchers, are considered cash and applied to the total cost of the ticket. You will see at the end of the stream of numbers and letters usually A/C, which stands for "Add/Collect" to reflect the amount of money you had to pay in addition to the vouchers. But the vouchers will not be reflected in the total price of the ticket. So in this example, ticket price $200, use a CS50, use $100 in VDB vouchers and $50 on your credit card. The amount of the ticket will be $150 with an A/C of $50.
I will refrain from addressing the ethical issue referred to in the original post.