Effective vassalization ways.

[Karaya]

How do you effectively vassalize a country?

I tried with relations improvement that usually stops at 189! instead of the required 190 opinion.

Tried combining royal marriage with granting independence, those seem
not to affect sufficiently their will to be vassalized.

I usually get a spot of time, when improving relations, during which i get the green check mark to vassalization, but it turns in a red X before they reach 190 opinion! : X

[[TFM]bobcat]

You can always vassalize through war. The way you do it otherwise depends on some factors. The factors that determine whether a vassal will accept your vassalage offer include the distance between you and the prospective vassal, your military strength vs their military strength, your economy vs their economy, your diplomatic reputation and their relative opinion of you. Usually increasing your military strength and relations are the easiest ways to make them willing to accept your offer of vassalge but increasing your economy by development investments or conquests is another, longer term method and the same for a distance negative modifier.

As for how to get relations to over 190, there are a number of other means you can use other than the ones that you mentioned. Apart from alliance, royal marriage and improve relations you can proclaim a guarantee, which will draw you into a war to protect them similar to how an alliance works but they would not be required to come to your aid if you get declared war on. This can be paired with an alliance even though the 2 of them combined are a bit redundant. This gives you +20 relations with them. You can also, assuming they have enough interest/use for it, offer them access to your country, which would give them +10 opinion. You can give them a gift (cash) which, depending on how much you give, can be up to +25 opinion, and lastly you can give them subsidies. This will not give you the full opinion all at once but will accumulate each month depending on how much monthly subsidy you offer and can give you up to +15 opinion

[KaykaysNL]

killing their rebels
military access
sending gift
sending subsidies
rival/attack their enemies/rivals
all give a + to relations
and not sure but i think sharing maps might also.

[Kapika96]

It should tell you the exact reasons why they will/won't accept.

The issue is probably that their development has increased though and now you aren't significantly developed enough to be able to diplo-vassalise them.

190 isn't hard though, you can also influence them for an extra 25 relations (+trust which gives extra reasons to accept vassalisation).

[Ayyas_Jackal]

More DLC = More way. (Transfer trade power, influence nation, take foreign debt... = DLC)
https://eu4.paradoxwikis.com/Relations
Look to this page for "List of modifiers" and "Improving relations" and mouse over their opinion at the game.
Alliance+Royal marriage+Guarantee = 85 Improved relations = 100. 185 positive modifier is easy without effort(and DLC). Send gift can solve your problem if you haven't any dlc.
Mouse over "offer vassalization" for your second problem. If economic power is the big malus: it is bad, you can't solve it without huge effort. If military power is the big malus: build army+navy, try vassalization when they have rebels, start a tactical war+invite them+allow enemy to kick their ass+peace+try vassalization...

[Karaya]

thx everybody