Many people have noticed that with Web Service Command Events, there is no way to tell the name of the Vault you are connected to. You can get a context to make API calls, but the Vault name is not part of the context. It's a limitation of the architecture.
So here is my suggested workaround: Create a Vault Option called "VaultName" and store the name there. That way, your code can call KnowledgeVaultService.GetVaultOption("VaultName") to get the Vault name.
Of course, the option has to be set up ahead of time, like during the install or the initial configuration of your app.
PS. Sorry for the short article. I'm busy finishing up the materials for my AU class on events.