I didn't even need to search for a way to grab holdings, the way to grab them found me! It's alsmost as if I'm in soviet Russia. My spymaster discovered a plot of the head of the Dandolo's to seize my tradepost in Zara. When you discover a plot, you can ask the plotter kindly to cease and desist. Or, you can throw him in jail. Well, you can throw any of your vassals in jail, but when you've discovered a plot, nobody will care. You don't want to be know as a tyrant as long as that isn't necessary. Anyway, jail. If you want to throw someone in jail, there's a fat chance he will rise in rebellion.
And thats A Good Thing(tm). Why, you ask? Because after you beat them senseless, you can revoke their titles without penalties! So that's how I got the city of Zara.
If I didn't have pics to prove it, you wouldn't believe that the head of the Ziani's was just as stupid:
I'm assuming Spalato is Split, just like Zara is Zadar. This is easy, I've increased my demesne by two without waging war. My personal economy is looking better now as well. What's bugging me is that my vassals can start "seize tradepost" plots, but I can't. I've found that you can only use this on families who own more tradeposts than you do (so tough luck for me).
Having all these high intrigue characters at court makes plotting easy. I've killed all family heads, and I've been so succesfull that for some families there aren't any adult males left:
And this is what happens if you kill the last one:
A bit of a pity, I had hoped to inherit his tradepost as well.