Your recipe doesn't specify 55°C, and I'd be surprised if 5–8 minutes over barely simmering water only gets that hot. Indeed, checking for sources:
McGee, in On Food and Cooking, says:
When the temperature reaches 120°F/50°C, high enough to unfold some of the yolk proteins, the mix thickens, traps air more efficiently, and begins to expand. As the proteins continue to unfold and then bond to each other, the foam rises into fluffy mounds. The key to maximally light zabaglione is to stop the heating just when the foam teeters on the cusp between liquid and solid. Further cooking will produce a stiffer, denser, eventually tough sponge as the proteins over-coagulate
so that leads to the conclusion that 55°C wasn't hot enough, as it hadn't yet reached "the cusp between liquid and solid".
Hans-Dieter Belitz, Werner Grosch, and Peter Schieberle say in Food Chemistry:
Egg yolk can be whipped into stable foam only at higher temperatures (optimum 72°C), the volume increasing about sixfold in the process. Above the critical temperature, the volume falls and the proteins coagulate. The protein coagulation is prevented by reducing the pH value, e.g., by the addition of acetic acid. This effect is used in the production of highly stable sauces
so that'd imply you want 72°C, or maybe higher due to the acid (wine) present. As a side benefit, that'd also pasteurize the eggs.
In summary, I think you should have cooked it hotter.
I doubt it is going to firm much in the fridge, hopefully it is stable enough to not further liquify.
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
You can replace the granulated sugar with confectioner's sugar, but due to that starch, you'll end up with an icing-like consistency. In fact, royal icing is just egg whites and confectioner's sugar.
There is actually a surprisingly large number of variables when it comes to beating egg whites, with the amount and type of sugar being just one of them. I'm not sure if this is going to be a complete list, but here's the basics of what you need to know for beating up to the "stiff peak" stage (and if you want soft peaks, then just don't beat for as long):
Age of eggs. Older eggs will whip faster and to a greater volume, but will be less stable (i.e. will deflate faster). Most people seem to find they get the best results with eggs that are about 3 days old. Very fresh eggs are difficult to whip, and very old eggs (more than 1-2 weeks) are just too unstable to work with.
Temperature. You want to separate the eggs when cold, because the whites are firmer and it will be easier for you to cleanly separate the yolks from the whites. But you always want to allow the whites themselves to come up to room temperature before you start beating them, for the exact opposite reason - the proteins are less firm, more stretchy, and will more readily accept air.
Impurities (especially fat). Make sure you don't have any impurities. Use a clean, dry mixing bowl and whisk. Don't let any of the yolk get in; even a trace amount will significantly decrease the amount of air you can incorporate, and if you happen to contaminate the whites this way, it's probably better just to start over.
Salt and acid both help egg whites to coagulate, which you might remember if you've ever had to poach them. That has a stabilizing effect which will allow you to whip the eggs longer and firmer without blowing a hole in the web of proteins, and will also help to prevent (or at least slow down) the foam/meringue from deflating afterward. You don't need more than a pinch of salt, and the traditional amount of acid is either 1/8 tsp cream of tartar (preferred) or 1/4 tsp lemon juice or vinegar. You'll want to start adding these early, as soon as you see the egg whites starting to foam.
Finally, sugar is critical to attain the stiffest peaks, but it is critical that you do not add it too early or too quickly! Sugar serves a similar role here as it does in baking; it dissolves in the water, absorbing it and drawing it away from the proteins, which improves stability a lot. But this particular type of stabilization is a double-edged sword; if you add it too early, it will interfere with the coagulation process and it will take forever to beat them to peaks. And if you dump in all the sugar at once, it will just deflate the foam you've worked so hard to produce (and possibly break it, requiring you to start all over again with new eggs).
In general, you want to add the sugar right after you see soft peaks beginning to form, and incorporate it very gradually while beating. If you want the final result to be soft peaks instead of stiff peaks (which you do, in this case), you can add the sugar slightly early, but err on the side of caution here. Stiffer peaks won't harm your pudding that much, but no peaks definitely will. I also suggest using superfine sugar, because the finer the granules, the quicker they will dissolve; but don't use icing sugar unless you want icing.
Finally, don't over-beat the whites. As I alluded to above in the salt/acid section, the beating process involves stretching the proteins and incorporating air; incorporate too much air and you'll just break the proteins, and then you'll have to throw the eggs out and start all over again. This is especially easy to do accidentally if you use cold eggs.
If you keep all of these things in mind then you can whip the eggs all the way to a nice stiff meringue; soft peaks should be a piece of cake.