I'm using Vault 1.2.6, mChatSuite b63 and CraftBukkit 1.1 R1 (RB). Vault doesn't seem to find mChatSuite at all. I have tried these plugins on a test server without any other plugins with no results.

looks like mChatSuite is not mChat, so this is expected. Vauly doesn't support mChat Suite yet, just mChat