Fixing VPK mounting for older Source SDK Bases

==Background==
When Valve rolled out the Steampipe update and the existing titles were updated to use the VPK file format, Source SDK Base and Source SDK Base were supplied with "depot" VPKs, but were left unable to properly mount them. Old mods using the Bases were often broken, and 2007 Base mods were unable to reference Half-Life 2: Episode Two content, unless it was directly copied into the mod's folder; this is despite the fact that the depot VPKs included those files (but were switched around; see below).
Apparently, this wasn't meant to happen, and both SDK Base 2006 and SDK Base 2007 can mount their VPKs with a specific fix.
==Overview==
The gmod9.com blog details an extensive research into this issue and provides a working solution.
It explains that the Steam2 wrapper, meant to handle VPK mounting for the old games, requires a missing file, , which is not shipped in any known product and whose structure needed to be reverse engineered.
There are three known methods to fix it, starting from the easiest one:
==Creating custom steam_vpks.vdf== 
The file in question needs to list relative paths to the depot VPKs. 
Create it in the Base's root (e. g. for '06, for '07). You can use Notepad++ (or basic Windows' Notepad) to edit it.
The following example lists depot VPKs for Base 2006:
"steam_vpks"
{
	"spewfileio" "true"
	"mount"
	{
		"vpk" "vpks/depot_206"
		"vpk" "vpks/depot_207"
		"vpk" "vpks/depot_208"
		"vpk" "vpks/depot_212"
		"vpk" "vpks/depot_213"
		"vpk" "vpks/depot_215"
		"vpk" "vpks/depot_308"
		"vpk" "vpks/depot_309"
		"vpk" "vpks/depot_381"
		"vpk" "vpks/depot_421"
		"vpk" "vpks/depot_422"
		"vpk" "vpks/depot_423"
		// other vpks can be mounted here as well
	}
}
 
*The "vpk" keys point at files relative to the Base's root folder (as you can see the folder right inside the root).
*The spewfileio parameter defines if additional debug info is being spewed; usually you can leave it at .
This file allows the Base to load its depot VPKs and, in turn, allows a mod for that Base to properly initiate and reference content. This works without needing to change the mod's or moving it to another Base.
===Testing===
A simple way to check it is to try and load an old mod for this Base without the file present (or renamed to anything other than ). The typical behavior is that either the mod would crash and report being Unable to load manifest file (typically or something similar); another frequent error is Can't find background image materials/console/startup_loading.vtf. Some mods can load, but then will be missing assets; this is frequently the case with Base 2007 mods that need Ep2 content. 
Now place the file you created, , next to the Base's . The mod should now properly initiate.
==Using mount_vpk function==
A relatively simple alternative in your mod's code. Calling with the path to a depot, relative to the Base's main directory, should get the target VPK at that path mounted.
==Using SteamMountFilesystem==
According to the blog, this isn't the best way, as it requires using specific paths to the depots -
<game_path>/vpks/depot_<depot_id>_dir.vpk
Nevertheless, calling this function in the same manner as filesystem_steam.dll does it, should work.
==Additional caveats==
===Difference in paths===
The Bases' depot VPKs follow a slightly different filing structure compared to all others. Their tree looks like (materials, models, sound, etc), whereas normal VPKs look like . 
Thus, even with the fix, they won't properly mount "traditional" VPKs shipped with the games - the resulting path to files would be treated as incorrect due to a 'missing' folder in it. 
It also means that while you can pack the mod's content into a VPK and reference that VPK in , you have to add a 'fake' game directory into the path:
root\fake_game_dir\maps
root\fake_game_dir\models
root_fake_game_dir\materials
etc...
and reference that folder in the mod's 's SearchPaths:
Game fake_game_dir
===Misplaced VPKs===
To make matters worse, SDK Base 2007 does not include files from Episode Two. Those VPKs - - are all shipped with SDK Base </u>2006</u> by mistake, even though 2006 is supposed to "reflect" Episode One.
A crude, but working solution requires having both SDK Base 2006 and 2007 installed at the same directory (same ) and referencing 2006's depots by their relative paths, i. e.:
"vpk" "./../Source SDK Base/vpks/depot_421"
"vpk" "./../Source SDK Base/vpks/depot_422"
"vpk" "./../Source SDK Base/vpks/depot_423"
===Suspected corruption of the VPKs===
=== Base 2007 doesn't draw menus ===
There is a bug that causes Base 2007 and its mods to not display the main menu, console panel, HUD or other interfaces. While they are functional (main menu buttons work and produce button sounds, the console can accept input), they are invisible and have to be navigated 'blindly'.
It appears to be caused by mounting . The reason is unknown; when unpacked, its contents don't appear to be causing the same issues (if any). It may be some kind of a corruption.
==Credits==
All info on researching and documenting this is taken from the gmod9.com blog.
The original credits include:
Thanks to Jai "Choccy" Fox for helping me with the article and the subject.
Category:Bug fixes
Category:Source
Category:Steam