Looks like I was wrong about the numbers today. It seems to be around 25%.
Homeless Veterans
Published by the National Coalition for the Homeless, September 2009
This fact sheet examines homelessness among U.S. veterans. A list of resources for further study is also provided.
BACKGROUND
Far too many veterans are homeless in America—between 130,000 and 200,000 on any given night—representing between one fourth and one-fifth of all homeless people. Three times that many veterans are struggling with excessive rent burdens and thus at increased risk of homelessness.
Further, there is concern about the future. Women veterans and those with disabilities including post traumatic stress disorder and traumatic brain injury are more likely to become homeless, and a higher percentage of veterans returning from the current conflicts in Afghanistan and Iraq have these characteristics.
The U.S. Department of Veterans Affairs estimates that 131,000 veterans are homeless on any given night [1]. And approximately twice that many experience homelessness over the course of a year. Conservatively, one out of every three homeless men who is sleeping in a doorway, alley or box in our cities and rural communities has put on a uniform and served this country.
Approximately 40% of homeless men are veterans, although veterans comprise only 34% of the general adult male population. The National Coalition for Homeless Veterans estimates that on any given night, 200,000 veterans are homeless, and 400,000 veterans will experience homelessness during the course of a year (National Coalition for Homeless Veterans, 2006). 97% of those homeless veterans will be male (Department of Veterans Affairs, 2008).
DEMOGRAPHICS
The U.S. Department of Veterans Affairs (VA) says the nation's homeless veterans are mostly males (four percent are females). The vast majority is single, most come from poor, disadvantaged communities, 45 percent suffer from mental illness, and half have substance abuse problems. America’s homeless veterans have served in World War II, Korean War, Cold War, Vietnam War, Grenada, Panama, Lebanon, Operation Enduring Freedom (Afghanistan), Operation Iraqi Freedom, or the military’s anti-drug cultivation efforts in South America. 47 per cent of homeless veterans served during the Vietnam Era. More than 67 per cent served our country for at least three years and 33 per cent were stationed in a war zone.
Here are some statistics concerning the veterans homeless [2]:
23% of homeless population are veterans
33% of male homeless population are veterans
47% Vietnam Era
17% post-Vietnam
15% pre-Vietnam
67% served three or more years
33% stationed in war zone
25% have used VA Homeless Services
85% completed high school/GED, compared to 56% of non-veterans
89% received Honorable Discharge
79% reside in central cities
16% reside in suburban areas
5% reside in rural areas
76% experience alcohol, drug, or mental health problems
46% white males compared to 34% non-veterans
46% age 45 or older compared to 20% non-veterans
Female homeless veterans represent an estimated 3% of homeless veterans. They are more likely than male homeless veterans to be married and to suffer serious psychiatric illness, but less likely to be employed and to suffer from addiction disorders. Comparisons of homeless female veterans and other homeless women have found no differences in rates of mental illness or addictions.
http://www.nationalhomeless.org/factsheets/veterans.html 
