Recruit with Marshall (MTTH 7 years as count, 1/3 of that if higher rank).* It's far better to raid with event spawned raiders as no need to pay maintenance on them. So if there's a direct vassal count, he'll be pretty helpless to your raiders, but a super duke may pop up some thousands to resist you.

Note that even vassals will raise troops to defend their holdings from raiders without waiting for the top liege to form an army, but they only do this in their own subrealm. Also, try to target holding that already have the levied raised so there are less defenders. The best places to sack are prince-bishoprics because you won't have to siege a castle first and likewise a county where the holdings go church-city-castle allow to get in two quick sacks without having to wait forever to siege the castle. As such, tribal holdings aren't usually worth the cost of the raiders, but if there are potential hostages there, you can still turn a profit albeit less reliably. The amount of gold you get from sacking is scaled to the tax income of the holding, which is in turn based on holding type and constructed buildings. If you do finish the siege, you will sack the holding for some bonus gold, a chance to imprison courtiers residing there, and there is a small chance to destroy a building or even the entire holding. Once the loot bar is fully depleted, or at least down to the protected level, you can move on if you don't want to keep sieging. Obviously, they will conduct a siege while doing this whether you intend to actually finish the siege or not simply by virtue of being parked in a hostile province. They'll collect some gold every four days. You need to keep the looter army parked in a province for them to loot.

Crusader kings 2 how to raid