You would need to create different "security domain" for each VDB, and configure VDB to use that security-domain. See  for setting security domain property in vdb.xml file.
As per creating the different security-domains in WildFly, see 
Can we create security domain programatically/dynamically? using using teiid admin api or any other api?
Using the AS cli, you can create security domains. See the snippets from the Teiid scripts - teiid/teiid-feature-pack/wildfly-integration-feature-pack/src/main/resources/content/bin/scripts at master · teiid/teiid…
For example in standalone mode - teiid/teiid-standalone-mode-install.cli at master · teiid/teiid · GitHub
Each security domain would need its respective login modules configured so that users were segregated.
If you are looking for some other mechanism, such as restricting vdb access based upon role, there isn't something like that yet but you can create an enhancement request if desired.