This is not formally a police chief so, I would go for the answer provided

1. Questore is not, or at least not necessarily a commissioner.
2. Aggiunto is not, or at leat not necessarily second. May be third, forth, etc.

Would be tempted to provide an old-fashioned "ajunct deputy quaestor" – and why not, eh?

I'd rather ignore the fact that it has a similar name to a Police rank. In my opinion, using a police rank may lead the reader to think it is a Park Police, like the one in the USA, instead the Forestale is a broader-scope police force.
Also remember that UK police forces are completely different from the Italian ones, for one thing all of Italian Police depends from the Ministry of Interior, local chiefs of police are appointed by the central government and have jurisdiction all over Italy (a Rome cop can be a cop everywhere else in Italy, in fact almost all police officers change AT LEAST two cities in their career, especially if they are good), while in UK (and in USA even moreso) police forces are more "local".
Formally, the Vice Questore aggiunto corresponds to a Lieutenant Colonel (tenente colonnello) in the Carabinieri and the army (see reference I posted).
Military ranks (especially officers) are more immediate to understand in my opinion.


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Note added at 6 hrs (2009-02-01 23:10:08 GMT)
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To kommos:
The legge 1° aprile 1981, n. 121 officially fixed equivalence between the "qualifiche" (which, I repeat, are ranks, not positions) of police, correctional officers, Guardia di Finanza and Carabinieri's ranks, which are military in nature. So, using Lieutenant Colonel is ok for Vice Questore Aggiunto. The only objection is that Corpo Forestale is not a military force but that is actually a minor distinction. Police forces all around the world use military ranks (US Police forces, for example) so that is actually not a problem.
Like I said, I chose a military rank because it is easier to recognise and understand throughout the English speaking countries. A US citizen may not be familiar with the term "Commissioner" (although the NYPD has a Commissioner, but it is not a sworn rank like Vice Questore Aggiunto) and these are usually very high ranks.
But at least commissioner is not tied to Police and MIGHT be taken as a rank.

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Note added at 6 hrs (2009-02-01 23:19:57 GMT)
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To Kommos:
The position of Mr.Giovanni Misceo is unknown and of no concern. Vice Questore Aggiunto IS ONLY A RANK. There is no such thing as a position of Vice Questore aggiunto, yes, there is Vice Questore which, conversely, is ONLY a position and not a rank.
And besides that, CFdS is a Corpo Armato dello stato, and yes, it is also a police force but you cannot call it Police, because it is not police. At least say Forest Service or Forest Police (which is incorrect as CFdS is much more than that).