From the Galaxy press release on the trade:

The LA Galaxy has confirmed today acquired defender Greg Vanney from D.C. United in exchange for midfielder Quavas Kirk. Vanney, who began his MLS career with the Galaxy in 1996, returns to Los Angeles after previous stops at FC Dallas, Colorado and D.C. United, as well as a three year stint with Bastia in France’s Ligue 1. A native of Tempe, Ariz. and a graduate of UCLA, Vanney was out of contract with United but has signed a new deal with the Galaxy. Per league and team policy, details of the contract were not disclosed.

“Ruud and the entire coaching staff are pleased to be acquiring a true professional such as Greg, who is coming off a great season with D.C. United in which he helped the club capture the Supporters’ Shield,” said Galaxy Director of Soccer Paul Bravo. “Greg is coming back to our club a stronger and more experienced player, who will be able to play multiple positions in the backline.”

The 33-year old Vanney has played in 245 career regular season games during his first nine years of MLS, scoring 22 goals and adding 22 assists, as well as five goals and six assists in 32 postseason matches. During his six seasons with the Galaxy (1996-2001), Vanney helped LA reach the MLS Cup three times, as well as leading the club to the 2000 CONCACAF Champions Cup and the 2001 Lamar Hunt U.S. Open Cup Championship.

“It’s great; I am excited to be back. I left six or seven years ago and I always felt like this was where I belonged,” said Vanney. “Though I have been through several experiences, my heart has always been with this club. I am looking forward to bringing trophies and championships back to LA.”

After leaving the Galaxy following the 2001 campaign, Vanney signed with Bastia of France’s Ligue 1, appearing in more than 50 games, mostly at right back, during his time at the club. He returned to MLS for the 2005 season, joining FC Dallas, with whom he played two seasons, appearing in 54 games before being traded to Colorado prior to the start of the 2007 season. He split time last season between Colorado and D.C. United, appearing in a total of 24 games, making 23 starts at both center and left back.

Vanney made 37 appearances for the U.S. during his 10-year National Team career, helping the Red, White and Blue qualify for the 1998, 2002 and 2006 FIFA World Cups. Vanney was named as an alternate for the U.S. squad at the 2002 World Cup in Japan/Korea and was briefly added to the 23-man roster following a pre-tournament injury to Chris Armas, only to suffer a knee injury himself, ruling him out of the tournament.

Entering his fourth season in MLS, Kirk joins United after appearing in 33 games for the Galaxy, making 15 starts in his three seasons with the club. Kirk scored his only ever goal for the Galaxy in their 2-0 win over Roma FC in the Fourth Round of the 2006 U.S. Open Cup. The 20-year old Illinois native battled foot injuries for most of last season, limiting him to just 16 games in all competitions.

Meanwhile, Harden's exit was indeed so he can concentrate on "philanthropic endeavors away from professional soccer."

The (edited) Galaxy press release:

The Galaxy’s 2007 Visa Defender of the Year, Harden made 24 appearances, all starts, and played 2,086 minutes with the club.

"While it's difficult to lose such a talented young player, we recognize that there is much more to life than soccer,” said LA Galaxy President and General Manager Alexi Lalas. “We respect Ty's decision and are happy that he will have the opportunity to fully pursue his mission of helping others. He remains a member of the Galaxy family and the door is always open for his return."

A native of Junction City, Oregon, Harden made a big splash in 2007, his rookie season. Despite missing playing time in the second half of the season with hip and calf injuries, Harden, 23, provided stability in the heart of the Galaxy defense after being selected by the club in the second round of the 2007 MLS SuperDraft. Tough and strong in the air, Harden battled some of the league’s top forwards game after game last year.

"My parents have worked for Goodwill for over 25 years, so it has been ingrained in me that helping others is something that we have been put on earth to do. It has always been part of my life and something that I wanted to do,” said Harden. “I have loved soccer for a long time, but I have always loved helping people and doing things for others. I just got to the point where I felt there is more that I can do than just playing soccer.”