Vathek as an ironic character
We might see Vathek himself as an ironic character, as his flaws are often dramatized to the point of humor. As such, we might see Vathek as an ironic representation of an "immoral person," and what that concept might look like pushed to an extreme.
The dark world
Again, there is something ironic about Beckford's portrayal of the antagonistic Eblis. For example, his dark palace is so excessive and typically "evil" we might see this character as ironic.
Vathek as an ironic text
Critics have often read the "moral" of Vathek as being ironic and sarcastic. Indeed, the novel itself is far-fetched in places, leading to this argument.
Vathek as a leader
As a leader, we might expect Vathek to care about his subjects. However, he actually is shown to be cruel and selfish as a ruler, even sacrificing children for his own interests.
The irony of the dark world
Vathek is thrilled when he finds the dark world and the palace of Eblis. However, this ironically turns out to be his downfall as he is killed soon after he arrives.