I usually don't have "real" sweeteners in the house at all (hubby keeps a small teddy bear of honey, but I'm allergic to most honeys -which means I'm probalby allergic to a specific, but common pollen).
We keep Equal and Sweet n Low (or their generic equivalents) in the house for hubby, but I usually use Splenda as my main sweetener in recipes calling for sugar, including nuoc cham. I didn't know the name "nuoc cham," but when I looked it up, I realized I make this all of the time (I use it as a dipping sauce, a salad dressing, a meat or vegetable marinade... Heck, I could almost drink the stuff).
Here's my recipe (though I never really measure).
3 tablespoons lime juice or rice wine vinegar (or combination of the two)
3 to 5 tablespoons fish sauce (depending on the strength of the fish sauce)
1 small garlic clove, finely minced (I'll sometimes use dried minced garlic in the winter. Fresh garlic is better, but sometimes winter garlic is bitter).
squirt of Sriracha or other chili or chili garlic paste/sauce
water and Splenda to taste (depending on how sour the lime/vinegar and how strong the fish sauce)