virgin
There are several reasons why AV Yveltal is generally not a good idea in this specific metagame. The biggest reason is perhaps its lack of power. With only AV (and not another item such as Life Orb), Yveltal struggles to muscle past common Pokemon such as Support Waterceus, Klefki, and Xerneas. Simply put, AV Yveltal is too weak to do any real damage to the opposing team, losing its niche as a hole-puncher. Secondly, AV restricts the use of moves such as Taunt. Taunt help with getting past Klefki, preventing a Defog (on switch-in), and in general is a useful move to have. With regards to your contention that the use of Assault Vest "counters" Darkrai, I wholeheartedly disagree. First, let me loosely define "counter." A counter is something that can switch into the aggressor (in this Darkrai) and can, in general, tank any of its moves and threaten it back. In this light, AV Yveltal is most definitely not a counter to Darkrai. If Yveltal switches into Darkrai, it can potentially be put to sleep by Dark Void, thus neutering its efficacy as a counter. If you do decide to sack another Pokemon to sleep, this means that Yveltal has not prevented Darkrai from doing its job, which in turn means that AV Yveltal is not a counter to Darkrai. However, this argument is based largely on semantics. It is true that AV Yveltal is a CHECK to Darkrai. However, much better checks exist (better in that they preform roles other than checking Darkrai). The aforementioned Mega-Diancie is a good check to Darkrai, as it resists its main STAB and can threaten it back with Moonblast. Diancie also provides some neat functions with Magic Bounce and is commonly featured on Sun Balance. Sleep Talk Ho-Oh is also a good check to Darkrai, as it beats virtually any set (aside from the incredibly rare Taunt variants) while simultaneously threatening half the tier with Brave Bird+Sacred Fire.
tl;dr: AV Yveltal fails in its prescribed functions and fails to do anything of note.