Originally posted by Jacob
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A word of caution though... I was playing as Denmark again (having fixed the explorer/conquistador thing so I'd be able to explore the world a bit) and I was pursuing the diplo-annex-Sweden strategy. As our relationship reached the +170'ies, the vassalization expired or was cancelled or something, which set everything back a fair bit as I'd have to ally with them then vassalize them again. That happened in 1522, I think... so, if you're gonna do it, do it quickly.
Jacob
They start out as Denmark vassal. But they will cancel the vassalization in about 10 years, you will know this when it says "Our royal marriage with Sweden has expired". As soon as that message appears, the vassalization is over.
Next problem. You try to vassalize them again around 1510-1520? No, this is a bad idea. Because in a few years, either you or Sweden will convert to Protestant.
As soon as one of you converts to Protestant, vassalization will break, your diplo-relations go down again dramatically.
Now you are back to square one.
Best advice for maximum success: try to vassalize Sweden AFTER the Reformation, after both you and Sweden have already turned Protestant.
This way, you can be sure that your vassalization will stay no matter what. All you have to do is to keep your relations high...