Why does the amount of CO2 produced decrease with more baking soda?
This graph is a very good example of experimental errors which may give rise to a pseudo-phenomenon i.e., an observation which apparently looks like a scientific phenomenon but it is an artifact of the experimental design.

First of all work out the moles of acetic acid (5% acetic acid) and moles of NaHCO3. As you can see vinegar has become a limiting agent. And it should have become a flat curve only if your system was a closed system. 

So where is the culprit in the experimental design?

As you stated "It was about a 1-1.5 second delay between poring the vinegar and capping it."

By the time, someone puts a syringe, a large volume of gas has already escaped when acetic acid was poured on a heap of sodium bicarbonate. How big is the heap of 3 g NaHCO3!

There is a nice trick to avoid these problems. You put the sodium bicarbonate in an open plastic pouch so that acid does not immediately encounter the salt. You can stopper and then shake to mix (quite well).

So your hypothesis is right for a flat curve, sadly there is an experimental design issue.