I never heard of vBulletin 3.7 or 3.8 existing yet
At the moment there's vBulletin 3.0, 3.5 and 3.6 in the 3.x branch. Styles for an older branch doesn't have actual templates if they aren't using the default ones. This can affect some features and of course never features aren't integrated in the old templates yet (if they aren't using the default ones). In the worst case, there can be a security issue. If you have some vBulletin knowledge, you can update the templates yourself.
The changes between 3.6.7 & 3.6.8 templates are listed in the 3.6.8 release thread on vbulletin.com, off-hand I can't remember anything that would cause any major issues.