It’s not subterfuge, but you can ask other countries to become your vassals, you need very good relations, a military alliance, and a royal marriage. And it helps if your monarch’s diplomacy skill is very high, and even then it’s not likely they’ll agree. And asking and getting refused drops your relations, so you’ll have to build it back up again before you ask again. And once a country has been your vassal for 10 years and you have very good relations you can ask to annex them. Again, it’s very likely they’ll refuse, your relationship score will drop, you’ll have to build it back up, and try again. But it can be done. And note that diploannexing doesn’t hurt your reputation nearly as badly as military conquest, although each new province you get gives you a reputation hit…when I annexed Castile I nearly doubled the size of my country, and that shot my reputation through the roof. From an in-game standpoint that’s supposed to represent how scared the other countries are that I’m going to attack and conquer them, so when you take over another giant country everyone gets nervous, and rightly so. In some ways having a powerful vassal is better than outright annexation, especially if they have a different religion or culture or aren’t connected by land to your capital.
However, this doesn’t mean you can skimp on your military, because if your military is weak that means your neighbors will think it’s a perfect time to attack you. You’ve got to stay strong enough to deter attack. And you’re going to be attacked, although lots of times it’s your allies that get attacked, or attack their rivals and you’re swept up into the war. If you’ve got a minor vassal next to a major power, that other power is going to try to grab it up eventually, and then you’ll either have to fight or let your vassal be annexed. And there’s the matter of religion, countries with different religions will tend to be your enemy, and there’s no way to avoid this. Even if all your neighbors share your religion and are friendly with you, they’re going to border on unfriendly countries, and those unfriendly countries are going to attack your friends, and if you don’t help your friends they’re going to get their asses kicked, and then the borders of those unfriendly countries are going to creep closer and closer.