If your received a deferred sentence and the case was eventually dismissed, there is no way to delete that from your record. You legally can answer that you have never been convicted of a crime, even though the cahrge will likely appear on a background check.
There is no need to try and vacate anything, since the case already is a dismissal. Vacating a conviction only allows a prior conviction to appear on your record as a dismissal. From what you stated, it already shows that way.