Eggs – How to get Italian meringue to thicken

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:

• Beating Egg Whites with Granulated Sugar Added for Tapioca Pudding
• How to minimise sugar in meringue
• Why do my egg whites separate after whipping?
• Making my meringues form peaks
• How can I keep meringue pie-topping from falling?

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.)

Make sure you add the Hot sugar syrup while still whisking and whisk the whites till they are cool the sugar syrup cooks the egg and kills any salmonella in the eggs