How did you setup the sharing for VM.
You mentioned the word "fresh slave".
The sharing requires to replicate an existing website.
So in your JMS Template rule, you need to select a website to replicate and specify that instead of replicate the VM tables, you want to share it.
If you have purchase a JMS 1.3, see the rule like "shareMasterDomain" where you can find that the website selected is the master.
Perhaps you could review the tutorial video 16 that show how to share the users.
This is exactly the same with VirtueMart.
www.jms2win.com/en/tutorial#jms12x-16