Surely it's possible.
ACP / vBulletin Options / vBulletin Options / Forum Display Listing Options
Hide Private Forums = No
Then the Forum will be listed, but of course you can't get in if you do not have permission.
Now setup Registered so it does not have access.
Create a Usergroup Full Access that has access.
Create a Promition from Registered to Full Access when the user has at least X posts.
Done.
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