Of course, none of these sites show WHY the flag is being flown today. I thought I heard something on the news a day or two ago about Obama saying something about flying flags today for fallen soldiers or something--I'm not sure since I was only half listening, so I could be wrong.
"March 30, 2011 has been officially designated 'Welcome Home Vietnam Veterans Day' by the US Senate. It marks the 38th anniversary of the withdrawal of US combat and combat-support units from Vietnam." I found this info on historynet.com.