Washington's statement is not binding on the feds. They can define "conviction" differently from the State of Washington and refuse to honor it. We face that issue in deportation cases all the time. A state court order vacating a conviction based on rehabilitation or to avoid deportation consequences of a conviction will not be respected by Homeland Security. Many years ago, this was also the case with firearm rights as well, but Congress statutorily overruled the US Supreme Court ruling.
What further complicates this mess is that they don't always catch sheltered convictions so many of us know people who have a sheltered conviction and a Nexus card.
Lastly, there is at least one Washington Attorney General's opinion that confuses me. It seems to say that the someone who has their conviction vacated may not be able to own a gun:
http://www.atg.wa.gov/AGOOpinions/op...0#.US5_x6V82fQ
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Additionally, Washington offenders with vacated domestic violence convictions can't own firearms under the federal law because they are still convicted within the meaning of the Lautenberg Act:
http://www.washrecord.com/Fed%20v%20WA.htm