What are the exact steps and supporting documents needed to get my non-EU wife a Schengen visa?

I heard I can get my wife a schengen visa free of charge with no documents except our marriage certificate, is that correct?

More or less, yes.

Even if I have to apply through VFS?

The country processing the application must either make arrangements with VFS to process the application free of charge, or they must give you the opportunity to bypass VFS and apply directly at a consulate or embassy. As far as I am aware, different countries handle this differently.

I've just had a quick look at the embassy's website and that of vfs, and there's no obvious information about this. I would start by asking VFS about it; if they cannot help you then ask at the Italian embassy or consulate.

If the person you speak with does not know what you're talking about, you can say that you want to apply under Article 5, paragraph 2 of the freedom of movement directive, 2004/38/EC, which says in part that

Such visas shall be issued free of charge as soon as possible and on the basis of an accelerated procedure.

As to the supporting documents, you basically need to prove only two things, so you should require only two documents in addition to your wife's passport:

1. That you are a UK citizen, for which you will need to submit a copy of your UK passport, and

2. That you and your wife are married, for which you will need to submit a copy of your marriage certificate.

The official Schengen visa application form notes that some of its questions are not relevant for those who will travel under freedom of movement, so you should review the form carefully and skip those questions.