>> You start with 99% and dilute it down to 80% Be careful.

> I still don't understand the rationale for diluting.

There isn't one.  

The 99% may cost more, and there's no particular reason to choose it over
the 80%, other than the need to keep the 99% warmer than 60F to avoid
solidification.  
There was never a lot of call for the 99% back in the days of film
photography, as most of it was going to end up being diluted down to 5% or
so for photographic "stop bath", anyway.

The "be careful" advice is another very good reason to NOT dilute 99% to
80%, as it implies more handling of the chemical, and more chances to splash
it on oneself.
There's just about no way to get an exothermic "boiling splash" reaction
from adding water to acetic acid, but the "rules" about acids exist for good
reason.

In this case, I'd be far more concerned about splashing the stuff on your
hands or arms when pouring the acid out of the container, so I'd suggest
that the less handling, the better.
It's always a vile vial.

We were told in school to always add the acid to the water, NEVER pour water
into acid. ("A&W Root Beer", as high-school chem teachers intoned, or in
college, "Drop acid, not water")
But these chemicals are not explosives and are difficult to get much
"boiling" out of doing it wrong - if you check YouTube, you can see several
videos about how insignificant the "reaction" is, even with the exothermic
reaction of sulfuric acid, which is a much, much stronger acid than Acetic.

Regardless, safety glasses, gloves, long sleeves and some slow-motion
handling would be a very good idea, even with acetic.
I think that TV series about the teacher who made meth has taught more
practical lab safety than any other approach ever attempted.
I was mixing up some oxalic acid solution to decontaminate packages we were
picking up for newbees by pouring a small bottle of pre-diluted oxalic into
a  gallon sprayer jug of water, and  "STOP!" was yelled at me.
The person yelling was a fan of that show, and was worried that I was mixing
acid and water without a full set of safety gear, bless her heart.
(Come to think of it, oxalic is much stronger than acetic, even if oxalic is
a carboxylic acid.)

To define "Strong" vs "Weak" - Strong acids almost completely dissociate
into ions, weak ones don't.  Breaking those ionic bonds makes heat, sudden
heat can be "explosive".
The concern with strong acids like sulfuric acid is that the first few drops
of water will react with the acid instantly, and the heat from the reactions
COULD boil the water droplets instantly, which could spray acid out of the
beaker. No one wants an acid burn.  Pro tip - use an ice bath when diluting
acids, so that the exothermic heat simply heats up the cold solutions.

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