I had this same question on protocol and could not find a definitive answer anywhere. I wrote to the Coordinator or the Veterans Day National Committee and received this response:
“Hi ,
Thanks for your support of our men and women overseas. I can say from experience, that it helps to know that people back home remember and appreciate your sacrifices while on deployment. You’ve posed a good question, one I receive quite often. By definition, a veteran is someone who has served in the U.S. Armed Forces. However, on Veterans Day, it is customary to acknowledge and reflect upon those who are currently serving, particularly during this time of war. So, by custom, it is perfectly suitable to send Veterans Day cards to active duty troops overseas.
Sincerely,
Matt Bristol
Coordinator
Veterans Day National Committee
Department of Veterans Affairs
810 Vermont Ave, NW (OO2C)
Washington, DC 20420”
"Expect people to be better than they are; it helps them to become better. But don't be disappointed when they are not; it helps them to keep trying."--Merry Browne