if it is still an active ferment, vinegar will stop the fermentation. Brine will work, I'm sure you are considering the added salt content. Water will work. Fruit juices might be something to consider. It will change the flavor, but that could be a good thing. Fruit juices will cut the heat both from the sugar content and also the volume.
Consider what kind of vinegar you might choose. Some have less 'bite' than distilled white or ACV. Rice and balsamic are milder.
If the product is too hot, can you add more non-pepper ingredients to stretch it out along with some kind of liquid? If you add fresh produce, it will need to be cooked/processed to make it safe. Fermented produce has the low pH, introducing fresh ingredients will mess up the pH, that needs to be considered.
I've done a ferment that was half fermented and half fresh. Cooked and bottled with the HFH method. Red Jalapenos, onion, garlic, red bell peppers,-can't remember what else- loved it!
Good Luck! post back what you come up with.
SL