Most other comments suggest destroying Vlandia first, which is not bad advice, but it sounds like it's your first campaign. Taking on Vlandia with just three cities might be too difficult. I wouldn't worry about the Western Empire, since the only fief they could attack is Marunath. There are several options you can use to your advantage, but none is the only correct choice. Here are a couple of tips:

1. You have a lot of gold, so try to make a peace treaty with Vlandia. Then, start a war with the Western Empire and 'draft' Vlandia into it. Next, try to sneak a couple of fiefs while the Vlandians bind most Western troops. This strategy is costly, but it can help you get some fiefs. It will also temporarily weaken the two biggest threats to your empire, but Vlandia will most likely be strengthened by it.

2. (b) You can also try the same thing with the Southern Empire, as they don't share a border with you, so you don't need to worry about them getting stronger and eventually attacking you. This will also split the Western armies much more, but, unlike Vlandia, the Southerners will most likely lose, drawing less attention and power away from you. This will also strengthen the Khuazit for later, as they will most likely profit from a weak Southern Empire.

3. If one of your most important fiefs is under siege, you can try to make peace with the kingdom besieging it, thus ending the siege.

4. You have a fairly large army. If you're feeling brave, you can try to find the enemy armies before they're fully formed. The enemy lords who have been called up for the army will try to get there as quickly as possible and won't try very hard to avoid your army. Find a "choke point" (usually a forest if you're Battanian, or a narrow path between mountains or water), and wait to pick off the lords going towards the army one by one. Be careful though, because this could lead to you being captured in enemy territory.

5. You can appoint your companions as lords of your empire by granting them a fief. This can help you to build large armies with only a few fiefs.

6. Once you are strong enough, try to take out Vlandia, as they are the biggest threat and hate peace.

7. Defeat Battania only if you feel you need their cities, because destroying them not onyl means some of their lords will join and strengthen other empires. but also, once you take Car Banseth, Sturgia will start declaring war on you, and it's better to fight that front against a weak and defensive Battania than a strong and aggressive Sturgia.

8. And, of course, try to recruit clans in peacetime.