When playing an offensive role, I agree that Entei needs ExtremeSpeed to stand out from the crowd of offensive fire types, in this role Sacred Fire isn't enough. However, this Entei isn't designed to play an offensive role, it is a check to many of the metagames top physical threats such as Bisharp (who does 96% max with +2 Knock Off) and Mega-Mawile (who does 72% max with +2 Sucker Punch) amongst others. Even Mega-Charizard X fails to OHKO with either Outrage or Earthquake at +1, dealing 92% max with Outrage and less (79%) with Earthquake. With such bulk, Entei can easily switch into physical threats and spread burns with Sacred Fire. With Roar, you need not fear opponents setting up on Entei either, dealing damage via entry hazards. RestTalk allows Entei to do this job repeatedly without being reliant on Sylveon or your choice of Wish passer, albeit not being ideal.
By contrast, what does an Assault Vest set give you? Assault Vest boosts special bulk that doesn't help against these threatening pokemon that this Entei is designed to check. What pokemon does Assault Vest help Entei check that it can't already with Choice Band or physically defensive? Only really Mega-Charizard Y and Volcarona, it is reliant on the inaccurate Stone Edge to do so. The metagame is very biased towards the physical side. Not to mention that Assault Vest prevents the use of anything that doesn't cause direct damage, meaning it misses out on Roar and RestTalk, both of which are very important in helping Entei repeatedly do the job it is designed to do. The above set takes 54% max from Mega-Charizard Fire Blast in sun.
The above set gives Entei another niche apart from spamming Choice Band boosted Sacred Fire that scares off most checks due to the burn chance. It allows Entei to take on a different role as a dedicated burn spreader and check to some of the metagames top physical threats including Talonflame, Mega-Mawile, Mega-Charizard X, Bisharp and others. With the above set, Entei can now fit onto defensive teams that prefer the benefits Entei brings over Heatran (Sacred Fire > Lava Plume, physically defensive > specially defensive, less weak to ground) and Mega-Charizard X (Sacred Fire + Roar > Will-o-Wisp) thus increasing its versatility in OU. Hopefully I have a few more people convinced.