Here is my view on this matter. If you take a vacation and you get sick, that is your problem. It is your time off, so if you get sick, that is in your own time. If you get sick while you should be working, you should get sick leave, not in your own time. You don't get sick leave if you get sick in the weekends either, do you? As for your wife having to spend all het vacation days for maternity leave, that is plain ridiculous...
Actually, even the time you do work is not because your employer requests it, but because you've made an agreement with your employer to do so. On weekends however, (for M-F jobs), there is typically no agreement. So, asking your employer for lost weekend time would assume that they granted you the weekend in the first place, which they did not. -you never granted it to them.
You ask your employer when you can have vacation. You make a plan, together with your employer, when you don't have to show up, so to say. Then, if you fall ill in that time, you are still doing whatever you want. You don't show up, since that is what you agreed upon. So, in short, you agree with the employer that you will be away. So when you spend the day being sick, you are still away, regardless of you being sick or not. I just found out that you also get a sick day if you are sick in your vacation in the Netherlands apparently. EDIT: Sorry, I was in a bit of a rush and just repeated half of your post :P