Johnson spurned two more chances as he just couldn't quite find the net and he was made to pay when Arnaud rifled the ball past Gspurning after being put through by Felipe Martins.
After half time, Di Vaio had two great chances to double his side's lead but was denied by Gspurning, and the Seattle defence who made a couple of good blocks to stop the Italian striker.
Brad Evans came inches within getting Seattle level but he saw his effort clatter the post and bounce clear after Johnson had found him on the edge of the box.
The woodwork was to deny the Sounders yet again as Johnson's powerful effort smacked off the crossbar and went over with 10 minutes to play.
Mario Roberto Martínez and Rosales had late chances but couldn't hit the target, as the visitors somehow held out for the win.