The one fact about the Vienna Convention known by everyone is that diplomats are not subject to the law of the country to which they are sent. But they are subject to the law of their own country. Now setting-aside the question of whether or not Anne Sacoolas has diplomatic immunity (which is already covered here), surely she can be prosecuted in the US, assuming that causing death by dangerous driving is an offence there? Is the impediment that it is an offence under (most) state law, and she was not in any US state?
Anne Sacoolas is the wife of Jonathon Sacoolas, a CIA operative working at the United States Air Force listening station at RAF Croughton.
The report that Jonathan Sacoolas is a CIA officer (and his wife, Anne Sacoolasa, former CIA officer) is based on the report from The Guardian, 2020-02-09.
That Jonathan Sacoolas doesn't work for the NSA is based on a NSA statement published by the Washington Examiner, 2019-10-11.
Neither she, nor her husband, had diplomatic immunity based on the Vienna Convention Diplomatic Relations, 1961 nor by the Vienna Convention Consular Relations, 1963.
Her husband had diplomatic immunity based on the 1995 agreement between the US and the UK for US employees (but not spouses) at RAF Croughton, treating him as part of the US embassy.
- Statement 2019-10-21: The Secretary of State for Foreign and Commonwealth Affairs and First Secretary of State (Dominic Raab)
To my knowledge, the exact text of the 1995 agreement has never been made public, so it not known under which (if any) conditions spouses were previously covered by this a agreement.
On 22 July 2020, the UK and US governments amended the arrangement to remove immunity from criminal prosecution for the family of US staff.
In this affair, the United States has simply chosen to ignore the criminal actions of one of it's citizens committed on foreign soil.