I received a very prompt response from VN's San Francisco office today via email. The way I understand it is that per VN policy, I will not need to show the original credit card if 60 days have passed between the online purchase of the tickets and the date of travel. Below is the exact text from the email I received from VN:
"And you don’t have to show the credit card again based on the date of purchase did exceed 60 days, following VNA policy."
Thanks for the feedback.