Does the comparison of 20% of suicides being vets vs. 1% of the population being vets have any materiality (weight, significance) ? You judge.

The data in the CNAS study (see notes 2 & 3) come from Facts about Veteran Suicide (VHA, updated April 2011); also from whence:

30,000 - 32,000 US deaths from suicide per year among the population overall (Centers for Disease Control and Prevention)

Approximately:

20 percent are Veterans (National Violent Death Reporting System).

18 deaths from suicide per day are Veterans (National Violent Death Reporting System).
Taking 32,000 (all US suicides) x 20% (vets) = 6400 / 365 days = 17.5. Pretty simple - and also pretty simplistic.

Consider this one, A "Suicide Epidemic Among Veterans"? (2008); first for some background on the death stats:

Veteran status has been part of the standard death certificate since 1939. The National Vital Statistics System, however, is a terribly decentralized system in which the states voluntarily participate. The federal government is a subscriber to that data and pays for it on a per record basis through grant agreements with the states. That data element, veteran status, has never been collated at the federal level. So while the data exists on an existential level it might as well not exist in any practical sense.
Only a minority of states supply vet data. Obviously, this is an area where Big Data would be useful - Come on NSA !

In order to exert some control over the study, one might look beyond vet & non-vet to age and gender classes. The "Suicide Epidemic" article looks to those distinctions, based on 2004 data from two public use databases managed by the Centers for Disease Control and Prevention's National Center for Injury Prevention and Control (this and that).

First take the 2004 data for all US suicides by age (per 100,000):

15-19 years, 8.2; 20-24 years, 12.4; 25-34 years, 12.6; 35-44 years, 15.0; 45-54 years, 16.5; 55-64 years, 13.8; 65-74 years, 12.3; 75-84 years, 16.3; and 85+ years, 16.4.
The rates for vets vary all over the place - 17 to 32 per 100,000.

Second, take the data for males (per 100,000):

15-19 years,12.6; 20-24 years, 20.8; 25-34 years, 20.3; 35-44 years, 23.0; 45-54 years, 24.7; 55-64 years, 22.0; 65-74 years, 22.5; 75-84 years, 34.8; and 85+ years, 45.0.
and, third, the 2004 data for white males (per 100,000):

15-19 years, 13.5; 20-24 years, 22.0; 25-34 years, 21.7; 35-44 years, 25.6; 45-54 years, 27.5; 55-64 years, 23.9; 65-74 years, 24.1; 75-84 years, 37.0; and 85+ years, 48.3.
So, for the 20-24 years cohort, we see for 2004: 12.4 (all); 20.8 (males); and 22.0 (white males). Looking at all 20-24 years males, over a number of database years:

1979 -- 26.5
1980 -- 26.8
1981 -- 25.7
1982 -- 25.2
1983 -- 24.0
...
1993 -- 26.5
1994 -- 28.0
1995 -- 27.0
..
2002 -- 20.8
2003 -- 20.2
2004 -- 20.8
The salient point - compare apples with apples.

Next up, JAMA, Post-service Mortality Among Vietnam Veterans (1987):

The post-service mortality (through December 1983) of a cohort of 9324 US Army veterans who served in Vietnam was compared with that of 8989 Vietnam-era Army veterans who served in Korea, Germany, or the United States. Over the entire follow-up period, total mortality in Vietnam veterans was 17% higher than for other veterans. The excess mortality occurred mainly in the first five years after discharge from active duty (rate ratio, 1.45; 95% confidence interval, 1.08 to 1.96) and involved motor vehicle accidents, suicide, homicide, and accidental poisonings. Thereafter, mortality among Vietnam veterans was similar to that of other Vietnam-era veterans, except for drug-related deaths, which continued to be elevated. An unexpected finding was a deficit in deaths from diseases of the circulatory system among Vietnam veterans. The excess in post-service mortality due to external causes among Vietnam veterans is similar to that found among men returning from combat areas after World War II and the Korean War.
From this study, we can conclude that the critical period for in-theatre personnel (for motor vehicle accidents, suicide, homicide, and accidental poisonings) is the first five years post-theatre.

Similar findings for deaths among Australian vets and non-vets, in the later period starting 15 to 20 years post service, are found in Mortality of National Service Vietnam Veterans (1997), including:

Suicide, which has been of particular interest in Vietnam veterans, was not significantly elevated, with a relative risk of 1.13.
This is an area where one should make haste slowly.

Regards

Mike