The other morning, I saw my father's face on the train. Dad died 11 years ago, but I came across a man who could have been my father had fate dealt Dad a different hand.

The first thing I noticed about the man on the train was that he was he appeared to be a Vietnam Veteran, and proud of his service. On the back of his dirty jean jacket was a huge Marine Corps patch and other military patches. One that particularly caught my eye said: "In Memory of the 58,000 brothers who never returned from Vietnam 1962-1975."

My dad was an Army Green Beret and returned from his three combat tours in Vietnam with a couple of bronze stars and dozens of memories he never talked about.

This thin little man on the train was about my dad's age, with long hair, a scraggly beard. He was carrying on a conversation with himself and swatting at an imaginary fly. When there was a lull in his conversation, I caught his eye and said hello. He mumbled back, but didn't look at me. He got off the train at the next stop.

It's not the first time I've run across a Vietnam vet on the street who looks as if he's down on his luck. But it's just a reminder this Memorial Day to remember not only those who have died serving our country, but also those who survived.

2.7 million Americans served in Vietnam. Many veterans were attacked personally by their fellow Americans who opposed the war. I remember my dad talking about being spat upon when he was in uniform. Combine that national rejection with horrific combat experiences, and many Vietnam Veterans still have mental health issues and they may never get the help they need.

An estimated 400,000 veterans are homeless at some time during the year. Most of those homeless veterans have some kind of mental health or substance abuse problem, according to the National Coalition for Homeless Veterans; one of every three homeless men who is sleeping in a doorway, alley or box in our cities and rural communities has at one time put on a uniform and served this country.

According to a Department of Veterans Affairs study released in March, nearly a third of U.S. soldiers returning from Iraq and Afghanistan are being found to have at least one mental health problem.

Will they too, become Missing in Action right in front of us?