I am no specialist in that so I might be completely wrong : I doubt the U.S. 24hrs cancellation possibility applies in your case. You were charged in JPY, not USD. So the point of sale of your ticket is basically Japan and not the USA an so I'm afraid the US rules do not apply in this case. Would you have purchased the same ticket on the U.S. website of an OTA like Expedia or equivalent, you would have been charged in USD and the transaction done "in the USA" and so you could have applied for refund within 24hrs (I think that's what the VN person meant when she told you that the rule applies only to OTA. Also don't forget VN does not operate any flights to/from the U.S.).
Anyway, good luck to you and I hope you'll find a way to sort it out. I say again that I think your only possibility is that you keep this ticket and your friend is joining you by another more affordable flight if possible at a similar time as the VN one.