Flail Expertise's second effect is "In addition, when you hit with a melee weapon attack using a flail and the attack lets you slide a target, you can knock the target prone instead of sliding it." Ardent Strike lets you divine challenge an enemy on a hit, and Commanding Vow lets you slide an enemy that you divine challenge. Would this combination work, or not because the slide isn't coming from the attack directly?
1 Answer
No, you cannot.
As you stated, Flail Expertise says:
when you hit with a melee weapon attack using a flail and the attack lets you slide a target, you can knock the target prone instead of sliding it.
However, when you hit with Ardent Strike, you do not get to slide them. Instead, you are simply allowed to place your Divine Challenge on them, which allows you to slide them with Commanding Vow.
Thus, it is your Divine Challenge that is sliding them, not your Ardent Strike, and therefore you cannot trigger Flail Expertise.