There are a lot of references to a "vassal module" on this website. Can somebody tell me what that is?

Thanks

Welcome Syphax and sorry for my late reply.

You can easily find and download Vassal software from here http://www.vassalengine.org/download.php

And this is a brief description of that software
Quote from Vassal website
Vassal is a game engine for building and playing online adaptations of board games and card games. Play live on the Internet or by email. Vassal runs on all platforms, and is free, open-source software.

A user (in the name of Bill Bennett) created a game module about Commands & Colors: Ancients so all our community is able to play it online (thank you so much to GMT Games for allowing him to do that).
You can play some skirmish game with casual player found online or even enter some tournament competiotion!

Moreover you'll be a little familiar with Vassal interface due to scenario images on CCAncients.net are created directly from Vassal game maps.

Users from this website also created loads of "Fan made" scenarios that are regularly published for free use with Vassal.

Let me know if you need further details or would like to meet online for a fast game.