Vacating a felony is authorized by RCW 9.94A.220. The requirements are:
(1) you must have completed your sentence and discharged under RCW 9.94A.220 (usually a certificate of discharge is issued after you've finished your sentence and paid off your financial obligations; this starts the clock on the timeframe to become eligible for vacating your felony);
(2) there must not be any pending criminal charges, of any seriousness level, against you in any court;
(3) your conviction must be "non-violent" (if your assault was third degree, then it's nonviolent; if it was first or second degree, you will never be eligible to vacate it);
(4) Since the order of discharge was entered, you must have spend a specific number of years in the community without committing ANY OFFENSE or crime that resulted in criminal conviction - 5 years for a class C felony, 10 years for a class B felony.