Andy, welcome to the Board, and happy collecting! I was born in Alexandria, and I was a Little Leaguer there in the mid-1950s when I first began collecting bb cards.
The only DC-area show worth going to for vintage cards is the "Chantilly Show" at the EXPO Center just off of Routes 28 and 50 in Chantilly, VA. I believe this show happens 3 times a year. The first show this year was the first weekend of April. The next show is scheduled for July 12, 13 & 14. FYI, there is NO admission charge on Fridays. Here is a link to the promoter's website:
http://csashows.com/
Val