Hi Cuong,
I never sent my wife any letters so I could not tell you anything. However, I once mailed a package to my wife 3 months prior to my coming to VN. That package did not arrive till 5 months later which I had came to VN and left

... Missing letters are common in Vietnam but missing all 17 of them is something I have never heard of. It has been more than a year and a half now and you would have looked into this matter long before. Anyway, it is typical or expected for mails to take about 2 weeks between the US and Vietnam.
If you are worried about it, you need to file a complaint with your local USPS office and try to get something in writing, have your wife do the same with hers in her local town although she might not be able to get anything in writing. Then bring them up at the interview. However, as many members here of have said many many times before, think big picture. Having 20 hand-written letters does not make or break your case. In fact, having too many hand-written letters might cause you more trouble than doing you any good. We are living in the 21st century where emails, text messages, cell phones and other means of e-communication are widely used, thus, presenting the CO with 3" thick of hand-written letters might trigger a red flag.
Just my 2 cents ... Let see what others here have to say. Good luck to you.