Like other U.S. military officers detailed to the National Security Council, Vindman normally wears civilian clothes in keeping with the council's dress requirements.
But in formally appearing before Congress, Vindman is required to wear his Army uniform, as he did at Tuesday’s hearing at the U.S. Capitol and at his initial, closed-door deposition three weeks ago.
"A soldier performing duties in an official capacity will normally be in uniform," Col. Kathy Turner, an Army spokeswoman, said in a written statement. "In cases where a soldier is detailed to an agency outside of DOD, the individual would follow the policies of that agency."
Another U.S. official said that Vindman was wearing the appropriate uniform when testifying before Congress.
"I understand now he wears his uniform when he goes in," to appear before Congress, President Donald Trump noted during a Cabinet meeting at the White House that occurred while Vindman was on Capitol Hill.
Trump also said, "I understand somebody had the misfortune of calling him 'Mr.' and he corrected them."
Trump was referring to when Rep. Devin Nunes R-Calif., addressed Vindman as "Mr. Vindman" drawing a quick request from the Army officer that he be addressed as "lieutenant colonel."
When he was later asked about that exchange by Rep. Chris Stewart R-Utah, Vindman explained that his request was based off "the attacks I have had in the press and Twitter have kind of eliminated the fact that either -- marginalized me as a military officer."
"Do you always insist on civilians calling you by your rank?" Stewart asked Vindman.
"Representative Stewart, I'm in my uniform, wearing my military rank. I just thought it was appropriate to stick with that," Vindman replied.
One of the ribbons that Vindman is authorized to wear on his uniform is the Purple Heart, an award given only to service members who have been wounded in combat.
Vindman earned that medal for injuries he suffered in Iraq in 2003 following the explosion of a roadside bomb that damaged his vehicle.
Above his ribbons he is also authorized to wear the Combat Infantryman's Badge that can only be earned by a soldier who has experienced combat.