In the following set of words, which is different than the others?

Question

Quizplus · Accepted Answer

Insufficient is an adjective used to describe when a quantity of something is inadequate but does not have anything to do with the lack of originality, interest, or flavor. Therefore, it is not similar to the other words in the set, since they are all ways to describe something lacking conviction or brilliance.
Lackluster is an adjective used to describe something that lacks in vitality or is uninspiring. It is similar to the other words in the set (excluding 'insufficient') because they are all ways to describe something that lacks conviction or brilliance.
Bland is an adjective used to describe things that lack strong features, and are therefore uninteresting. It is usually used with food (lacking flavor) or activities.
Insipid is an adjective used to describe things that lack flavor, are weak, or tasteless. It is usually used with food (lacking flavor).
Vapid is an adjective used in the absence of stimulus or challenges, denoting lack of interest or intrigue.

In the Following Set of Words, Which Is Different Than