This directory contains the VST 2 SDK, but it's not part of the repository because the SDK is not free software.
Here's what the (relevant) contents looked like in the final version 2.4 rev2 in 2006:
$ ls -lR vstsdk
total 0
drwxr-xr-x 4 user group 128 Jul 26 2009 pluginterfaces/
drwxr-xr-x 5 user group 160 Jul 26 2009 public.sdk/
vstsdk/pluginterfaces:
total 0
drwxr-xr-x 6 user group 192 Aug 25 2009 vst2.x/
vstsdk/pluginterfaces/vst2.x:
total 176
-rw-r--r-- 1 user group 16961 Jun 20 2006 aeffect.h
-rw-r--r-- 1 user group 64725 Jun 20 2006 aeffectx.h
-rw-r--r-- 1 user group 4044 Feb 10 2006 vstfxstore.h
vstsdk/public.sdk:
total 0
drwxr-xr-x 4 user group 128 Jul 26 2009 samples/
drwxr-xr-x 4 user group 128 Jul 26 2009 source/
vstsdk/public.sdk/samples:
total 0
drwxr-xr-x 4 user group 128 Jul 26 2009 vst2.x/
vstsdk/public.sdk/samples/vst2.x:
total 0
drwxr-xr-x 4 user group 128 Jul 26 2009 win/
vstsdk/public.sdk/samples/vst2.x/win:
total 8
-rw-r--r-- 1 user group 44 Nov 17 2005 vstplug.def
vstsdk/public.sdk/source:
total 0
drwxr-xr-x 8 user group 256 Nov 13 2006 vst2.x/
vstsdk/public.sdk/source/vst2.x:
total 248
-rw-r--r-- 1 user group 3115 Jan 16 2006 aeffeditor.h
-rw-r--r-- 1 user group 24787 Jun 7 2006 audioeffect.cpp
-rw-r--r-- 1 user group 11541 Jun 7 2006 audioeffect.h
-rw-r--r-- 1 user group 56569 Oct 5 2006 audioeffectx.cpp
-rw-r--r-- 1 user group 17371 Jun 20 2006 audioeffectx.h
-rw-r--r-- 1 user group 2275 Sep 5 2006 vstplugmain.cpp
VST2 is definitely deprecated and it's harder and harder to find these files. You can maybe find them in the VST3 SDK, which for some time contained the VST2 headers as well.