The final scoreline was no reflection on the balance of play, but a quality goal from Dean Cox and a second from skipper Nathan Clarke secured the Londoners victory.
Slade said: "I've always thought this is a difficult place to come and I wasn't very happy early on because we started sloppily.
"But we gradually grew into the game and it was a terrific goal by Dean. Four players were involved in the move and it was a fantastic finish.
"The second goal was a bit fortunate. Nathan's got a good header in, but I've just watched it again and it looked like their keeper had two hands on the ball before it slipped through.
"Vale Park is a big pitch and it is important for your wide players to play their part.
"I thought ours (Cox and Moses Odubajo) were excellent to help us to a very pleasing three points.
"We had to soak up some pressure at times and our goalkeeper Eldin Jakupovic is pleased the match is over.
"He was kept busy in the second half and made some good saves."