I've always been taught that baking is a science when it is compared to cooking. Cooking is very much 'to taste' and very individual. There are not as many things that can go wrong with a standard recipe in cooking, and you have a lot more room for creativity. You don't have to look at baking as that precise. However, unlike cooking, where you can add or subtract from a recipe with no real harm to physical structure, that does not hold in baking. For examlple, if you feel that there is too much salt in a recipe, cutting back can (and most likely will) have a cascading effect through the ingredient chain. You have to understand your ingredients and the effect they have on other ingredients. That is what makes baking a more precise science.
And in terms of flour, it is often the most 'ranged' ingredient. Depending on flour type, miller, altitude, water temp. etc., the amount of flour in a given recipe is always a guide. Again, you have to know your ingredients. You will never see 'one and an eight cup plus 2 TBSP hard flour' in a recipe, because it is so variable for many different reasons. Flour and water are the two most flexible ingredients, and are always variable.
Hopefully this helps, I am sure someone will come along with a more scientific explanation for all different ingredients, I am just offering an experience based answer to your question.
Also, you are right, do not mess around with baking soda and powder. If you do, you are asking for trouble.
I would suppose that the rule of thumb would be that the more exacting an ingredient is (tsp, quarter tsp), you don't want to change much.
First of all, I suggest reading through our other questions on meringues and general egg-beating, to rule out any issues with your technique regardless of sugar content:
There are more, but I'm going to stop here - there's already so much about the subject on this site that I'm not going to waste any more time going into details about that. Suffice it to say, make sure you're using the right eggs, the right equipment, and the right ingredients, and doing things in the right order.
More specific to this question, however, is the fact that the sugar in a meringue is not just to make it sweeter. Sugar is a stabilizer; it is a critical ingredient in a meringue, and regardless of what Splenda and its recipes may say, it's not a perfect substitute for sugar.
Most Splenda recipes I've seen still use some sugar (preferably superfine), just less of it. Every little bit helps and you don't need that much. If you're not going to use any sugar then you'd better use some other stabilizer like corn starch, otherwise it's simply not going to be stable (unless, maybe, you've done everything else absolutely perfectly).
By your description, it also sounds possible that you might be rushing through it; "soft peaks" should be fairly smooth, not foamy or bubbly, like so:
If you don't beat long enough to get soft peaks, or if you start adding the "sugar" too quickly, it will collapse. I prefer to err on the side of firm peaks, it doesn't change the end result too much and there's less risk of a collapse. On the other hand, if you massively overdo it (i.e. try to get it all the way to stiff peaks) without any stabilizer, then it almost certainly will collapse, and once it does, you cannot recover it, it's like trying to blow up a balloon that has already burst.
So, to summarize: Try to use some sugar, or at least some starch as a stabilizer once the peaks firm up. Make sure your peaks are somewhere between soft and firm before adding your sweetener/stabilizers - not earlier, not later; and add them slowly enough to properly incorporate - you don't want to see any crystals or bubbles at that point.
(From what I can tell, your meringues failed long before the piping stage, and it definitely wasn't the vanilla's fault.)
Best Answer
3 grams (contingent on the relative weight of the rest of the recipe) is not a big deal.
But, since egg whites are such a pain to measure, it's very simple in recipes like this. Beat your egg whites slightly, then weigh and remove three grams. Much easier.