Those aspects of "VDM" relating to language description were developed in the IBM Laboratory in Vienna
around a project relating to a complier for the PL/I language.
The description of PL/I is contained a Technical Report which I've had scanned 
(so these are rather large PDFs).
TR25.139 consists of two parts:
<ul>

<li>
<a href="Part1.pdf">Part I</a> is an 88 page "introduction": 
as well as giving the main items of the notation, 
the largest part (C) tries to give a way into the rather dense Part II</li>

<li>
<a href="Part2.pdf">Part II</a> is just 118 pages of formulae</li>
</li>
</ul>