If you wonder where a zombie got an adamantine armor, he got it from the poor bloke that got beheaded by a vorpal sword, who thought it was a good idea to wear adamantine armor against a vorpal sword.
Adamantine Armor turns any critical hit into normal hit.
Zombie has Undead Fortitude trait that gives it a chance to survive non-critical-non-radiant damage.
Arguably, zombie does not need a head to survive, so it can survive beheading by vorpal sword. The only concern is the extra 6d8 slashing damage that increases the Undead Fortitude DC, but at least the zombie army will only be susceptible to radiant damage.
Did I interpret the rule right? Will the poor bloke zombie survive his second beheading?