Because you're in HAN, you can book a Y ticket on VN and the economy lite fare (not super lite iirc) lets you refund[0] for a ~$20 fee (please double check your fare rules when booking). I cancel Y tickets all the time because the fees are so little compared to other carriers. You can just email [email protected] from the email on your reservation and it's really straightforward.

[0] yes credit card refund every time, not a voucher.