You can play games distributed as .vpk files by installing them in the addons folder of a mod. This is a bare bones mod you can install if you don't know how to make your own.

You can't use the in-game menu to start a remote coop game. Apparently, the Steam servers have a problem connecting mods. You can do it from the console, using the following commands:

On the host machine: map <mp_coop_mapname>
On the client machine: connect <host-ip-address>

Version 1.1 Update README, cleaned up unnecessary files

File Name: CustomMaps.zip
File Size: 30.69 MiB
Click here to download Basic mod for playing .vpk files

note: unfortunately it is not possible to use mods to play multiplayer games at the moment

lpfreaky90 wrote:
note: unfortunately it is not possible to use mods to play multiplayer games at the moment

She should know, she tested it with me. A clarification is in order, however:

I have not figured out how to play remote coop games using a mod. I don't know that it isn't possible, just that I can't do it right now.
You can play splitscreen coop with this mod. I included the coop map Discovery just to prove that point. It is distributed in a .vpk, and you can play it splitscreen.

I heard you can't do coop in a mod before, because valve didn't add coop support in mods.

I've updated the description to reflect the fact that you can play remote coop in a mod. You just can't use the in-game menu to start it. You have to use a direct connection using console commands.

On the host machine, who will be the blue ballbot, start the map with:
map <mp_coop_mapname>

On the client machine, who will be the orange eggbot, connect with:
connect <host-ip-address>

Thanks to lpfreaky90's assistance, we've confirmed that this works over the internet. Version 1.1 updates the README file, and cleans up some unnecessary files (yes, it can be even more basic than it was before!)

I'm trying to run the mod, but I keep getting this error message: > Quote:

Setup file 'GameInfo.txt' doesn't exist in subdirectory 'hl2'. Check your -game parameter or VCONFIG setting.

eat911 wrote:
Quote:

Setup file 'GameInfo.txt' doesn't exist in subdirectory 'hl2'. Check your -game parameter or VCONFIG setting.

Is the folder "CustomMaps" extracted to C:\Program Files\Steam\steamapps\sourcemods?

Just a wild guess, but the VCONFIG message may be because the mod includes my cfg/video.txt file. Try launching the game with the -autoconfig option.

I was able to get it to run by putting it in the portal 2 folder and setting the launch options (for portal 2) to -game CustomMaps

eat911 wrote:
I was able to get it to run by putting it in the portal 2 folder and setting the launch options (for portal 2) to -game CustomMaps

Does the addons functionality still work that way? Can you play "A Little Higher", for example?

KennKong wrote:
eat911 wrote:

I was able to get it to run by putting it in the portal 2 folder and setting the launch options (for portal 2) to -game CustomMaps

Does the addons functionality still work that way? Can you play "A Little Higher", for example?

Yes

eat911 wrote:
I was able to get it to run by putting it in the portal 2 folder and setting the launch options (for portal 2) to -game CustomMaps

I just learned about the "gameinfo.txt" message being an issue with Macs. I should update the readme file, but since I don't have a Mac to test with, I don't want to say something that I can't personally be accountable for.