I think we have an idea of who it could have been ... The anti-war left would be my guess
There was nothing in the media about this. One would assume that if park employees knew about it they would have reported it to the park police and the reporters who make their livings reading police blotters would have picked it up. The America-hating MSM would love to report on the desecration of the VN War Memorial, especially if they thought the usual array of traitors and anarchists were successfully involved.