The difference between the law allowing one to prohibit one's self from buying guns and the other controls is the self-banning law does not go into effect until 2021.
WTOP reports the legislation, Senate Bill 436, was sponsored by Democrat State Sen. Scott Surovell (Fairfax). The measure passed the Democrat-controlled state legislature "along party-line votes."
The Virginia Mercury refers to Senate Bill 436 as the "Virginia Voluntary Do Not Sell Firearms List," noting that it takes effect next year.
The Mercury notes:
The new law will allow anyone over 18 to add themselves to the list, designed to be kept confidential, by filling out a form with a copy of their photo ID and mailing it or delivering it to the Virginia State Police. Once a person is on the list, it will be against the law for them to purchase or possess a gun and unlawful for anyone to knowingly sell or give a gun to a person on the list. If someone on the list changes their mind, they can be removed after waiting 21 days.
Washington state adopted a similar law in 2018 and 13 people have used to the law to ban themselves from gun purchases.