Custom TAB 1 is an extra tab that can be added/editted via "Flexible VM2 Template" plugin (it comes built-in with LightMart)
Open the plugin and change it's content (and the TAB title)
The rests of the TABs comes with Virtuemart Officially. If you enable "Ask a Question" or enable "Show Reviews" through VM Backend, those TABs will appear there. If you disable them, they will be gone.
The error you mentioned, it is a known problem (but it is not a critical error -- you noticed this, because you are testing and playing around, clicking all the tabs etc.. no real users will encounter this error ever) but it will be fixed in the next release anyway.
Regards.
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