The best option remaining then is requiring masks for everyone. You don't need to worry about looking for wristbands across the room. Spotting a mask is far easier. 
 

Yeah, even if the unvaccinated got their first shots immediately, they wouldn't be considered "fully vaccinated" by the time Gen Con Day 1 happens (the course takes about 6 weeks from start to finish).

J&J is one jab plus two weeks.

True. It's still a minority of the vaccines administered in the US though.

Moderna would be six weeks (28 days between shots, plus 2 weeks).
Pfizer would be five weeks (21 days between shots, plus 2 weeks).
J&J would be two weeks.

So the cutoff for Pfizer would be next Wednesday, the 11th.

Masks now required.

I would argue this is actually counterproductive to public health in that, with the June update, people were INCENTIVIZED to get vaccinated.  Now, Gen Con has removed that incentive.  And, masks aren't solving this problem.  Vaccines are the solution.

(Plus, who wants to do a 4hr RPG in a loud room with masks.  It is often hard to hear what people are saying in the first place - even before we added the masks.  Regretting my event ticket purchases.)

WHY would they bury the August September health update at the bottom of an email about gencon online? I've got no interest in online Gen Con I almost deleted the email without going all the way to the end

I'm sorry but if masks are now going to be required I'd like a complete refund for all events, 4 day badge and hotel. Please post how we go about that.
This is the most ridiculous decision that the GenCon could have made. 

Thx for opting out and protecting the rest of us 

Vaccinated people can still both get and transmit the virus. 
It's belt and suspenders. Mask to cut down on transmission, vaccine to reduce both transmission and severity.

I'll still go, but I will likely bail on some of my events.  There is no way I'm going to wear a mask for fourteen hours a day, four days straight.  Eight hours is barely tolerable.

As I said above, I expected this. 

I do note that they are specifically reserving the right to require vaccines in order to attend. At this point though, it's too late to mandate it. Masks are an easy purchase. The vaccine except for J&J takes weeks to protect you. And I say this as someone who wanted a vax only convention. 

It's not too late to mandate it.

It is too late for people to make the decision whether to start a two-dose vaccine course based on whether or not Gen Con is mandating the vaccine.

I would argue that if someone has waited this long to get the vaccine, it's unlikely that attendance to a game convention is going to push them to get it.  Maybe, but I think that if you're that anti-vax, there isn't much that going to change your mind.

If the incentive to finally get a lifesaving vaccination is the chance to go to Gen Con....

Then again, see Toxic Rat's post above.