Temperature change and gas evolution for vinegar and baking soda experiment
Although your observations make it pretty clear that you are using enough sodium bicarbonate to consume the acetic acid, even with $1$ teaspoon of sodium bicarbonate, we can also do a rough calculation to validate this hypothesis. It's a "rough" calculation because we don't know the concentration of acetic acid in your vinegar.  

Vinegar is typically about $\pu{5\%}$ acetic acid by volume, but can be as high as $\pu{20\%}$. For our calculation, we will estimate that there is $\pu{5 g}$ of acetic acid in $\pu{100 ml}$ of vinegar, and use $\pu{6 g}$ as the mass for $1$ teaspoon of sodium bicarbonate, per your comment. Then, using the molecular weights of acetic acid ($\pu{60 g/mol}$) and sodium bicarbonate ($\pu{84 g/mol}$), we can calculate the following molar quantities of the reagents:  

acetic acid: $\pu{0.083 mol}$ per $\pu{100 ml}$ vinegar
sodium bicarbonate: $\pu{0.071 mol}$ per teaspoon of baking soda  

If our estimate of the acetic acid concentration is correct, it is close enough to a $1:1$ molar ratio with the sodium bicarbonate that you may not have noticed the difference in your measurements. In this case there is $\pu{17\%}$ more acetic acid than sodium bicarbonate. This means that the addition of excess sodium bicarbonate (i.e. $2$ teaspoons) should result in a $\pu{17\%}$ greater temperature difference, which would be $\pu{4.7^oC}$ rather than $\pu{4^oC}$. Since you are only reading your thermometer to $\pu{+/- 1^oC}$, these could be said to be the same and it is perfectly reasonable that you wouldn't have noticed the difference. It is not stated how the volume of evolved carbon dioxide was measured, but it would likely be even more difficult to notice this small change by the change in observed carbon dioxide volume.  

Summary, TL;DR:
Your observations indicate that a single teaspoon of baking soda is consuming all of the acetic acid present in the vinegar, so when additional baking soda is added, there is no more acetic acid to react with and thus there is no indication of further reaction. Making some simple assumptions about the concentration of vinegar and mass of sodium bicarbonate in one teaspoon allows us to confirm that this is the case, within the detection limits of your method (measurement of temperature change and volume of carbon dioxide evolution).