1. You cannot own a sea zone like you can own a province. But you can block trade and invasions by enemies through it if you have one or more ship stationed there(independent of the number of enemy ships in that zone). This applies to your enemies as well.

2. Assassins are leveled up by successful missions. If they keep getting caught, the probably run into border forts, spies or high valour targets. You can get better assassins by upgrading your taverns or building them in Syria (+2 valour, VI-only). The English also have a hero assassin: Guy of Grisbourne. The first assassin the English recruit after 1205 will be Guy of Grisbourne.

3. Vlad Tepes is born in 1430, so he will come of age at 1446. The first unit that is finished in or after 1446 will have Vlad as its leader. So make sure you finish at least one high quality unit in 1446, or else you might have Vlad leading a group of peasants (heroes will be automatically signed to the highest quality unit that is finished in the year the hero comes of age).