Beardguy, I saw your post about this soap in the April challenge thread, too. I tried to respond, but was on my phone and it just doesn't cooperate well with posting to this forum. So tonight on my computer, I can respond with some experience I have had using vinegar and running into some of these crumbling issues.
What I suspect is the case, as mentioned above is waiting too long to cut because that is what I found out in my experiments with vinegar in soap. But ALSO, you may also have used too much vinegar as well. I have also had vinegar soap (I'm using this term to differentiate soap wherein vinegar is an ingredient) crumble when I wait too long to cut. But I have also had it happen when using too much vinegar.
So I would suggest that next time try half as much vinegar as you used this last time. For example, if you used 50% vinegar and 50% beer, try using only 25% vinegar for the next batch. And still cut early, because it works better when you do.
BTW, the soap is still usable even though it may be shaped like various oddly shaped rocks. But it may also splinter if you drop one of them on a hard surface. You can successfully re-batch the soap if you so desire. I have also done that with soap where it was too brittle (from too much vinegar). It was interesting to see it cut into tiny little shavings with almost no effort. When I rebatch, I prefer to use a mixture of new soap batter and add in the old soap at about a 50-50 mix. That works well with this kind of brittle soap.
Regarding using vinegar with lard, I have done that and it was the most brittle soap of any of the vinegar-added soaps. I found that decreasing the amount of vinegar to a tiny amount (even less than 25% of the liquid) works, but I don't really think it's necessary at all to make a harder soap. But I encourage you to do your own experimentation to find what works best for you in your circumstances.