According to the "ISV Windows Vista Compatability Cookbook" at the link I gave above:
Application installers that attempt to replace, modify, or delete OS files and/or registry keys that are protected by WRP may fail with an error message indicating that the resource could not be updated. This is because access to these resources is denied.
I would not assume that any given DLL or OCX present in Vista as a protected resource will have a newer version than one you might try to install. Major version number are unlikely to change for VB runtime components, and on your development system you may have higher version numbers in the minor and build numbers due to hotfixes and service packs, especially as time goes on.
However...
Certain installers are "well known" to Vista. If these installer programs are detected and then they try to overwrite a protected resource (file, registry key, etc.) they will be silently spoofed by Vista. I.e. the installer will "think" it installed the item but Vista will prevent it from overwriting the existing item. This behavior only occurs for recognized installers that try to install something Vista recognizes as a "legacy application."
So in practice it is hard to say until we've had some experience. I hear reports of installs failing a first, second, even a third time... then finally working.
Ideally you will want to rewrite your applications to work properly with Vista (lots of rules here), have a proper application manifest to tell Vista you aren't some "legacy" application, and use an MSI installer package instead of a P&D Wizard or Inno, etc. package. Visual Studio Installer 1.1 is probably your best bet for VB6 programs.
Again, at the link I gave above there is a ton of information.