The official Marjory Stoneman Douglas public safety commission report is wrong on this, which writes that he put the ammunition vest on inside the first floor stairwell.

Seconds after the timeline of 2:21:16, the official report on page 25 states that :
''Cruz made an immediate right turn into the east stairwell. At the base of the stairs, he
loaded his semi-automatic rifle and put on a magazine-carrying vest (the vest did not have
any anti-ballistic qualities). While Cruz was loading his firearm, student Chris McKenna
happened upon Cruz in the east stairwell and saw Cruz with his gun. McKenna told
investigators that Cruz said to him “you better get out of here, something bad is about to
happen…he told me to run.” McKenna fled out of the east stairwell door of Building 12.''

For educational purposes we have taken the enlarged ABC footage and slowed it down, ending with a focus on page 25 of the MSD report,
and as clearly seen in the security camera feed and
contrary to what's written in the report, while again entering the
hallway, this time from inside the east stairwell, and after readying the weapon the suspect still had not yet applied the ammunition vest.

Revealing how the official report is potentially prone to more errors, remember, that this report was not written by highly adept researchers and investigators.

So, when did the suspected shooter most likely put the spare ammunition vest on himself?
In this rare segment of the video released the suspect is seen going up to the third floor and seen with the vest on in 3rd floor hallway of the 1200 building. https://www.youtube.com/watch?v=DWs2LeQdKLc
A camera still from the second floor was also released by the commission, in it can plainly see the suspect has previously shed the backpack and placed the vest upon himself.

The Broward Co. Sheriffs time line and that of the official report which pertains to the claim of using the CCTV recordings to create their descriptive timeline states that the accused shooter put the vest containing spare ammo on while also readying his rifle in the stairwell.

Yet the video excerpt itself reveals this to not be the case.

He must have taken a moment to ready the ammunition vest while taking the back pack off and pulling out a mask but apparently not using it as the report reveals, thus leaving an even longer extended period where the shooter must not have had the rifle ready to fire promptly.

The report reads as such, on page 26:
''After Cruz lowered the rifle, he took a knee in the hall outside of the alcove of classroom
1216. He removed a knit ski-mask and camouflage shirt from the bag and placed these
items on the ground, but he never put them on.
From approximately 2:21:48 to 2:22:10, Cruz’s movements were not recorded due to du
...
https://www.youtube.com/watch?v=ye3x94YN5ik