The Vasa or Wettin? achievement requires 波兰 to become an elective monarchy, and you the player to get a ruler of their dynasty on the Polish throne.
A straightforward way of ensuring a ruler of the player's dynasty becomes the ruler of Poland, is take the Support Heir. The alternative is hoping that some other nation, which also shares the same dynasty as the player's nation, does so. Supporting heir is a chance based monthly diplomat action, but you can increase the chance of success in a number of ways. Having positive relations with Poland increases the chance of success, with higher positive relations conferring a larger bonus. An additional factor which influences the chance of success is the supporting countries diplomatic reputation, with higher levels of diplomatic reputation conferring additional support heir chance. For this reason although only starting as Poland is prohibited for the achievement, nations such as 法兰西, which have national traditions or ideas which increase Diplomatic Reputation will have advantage when attempting this achievement. Similarly completing missions which have positive diplomatic reputation as a reward, and hiring an statesman advisor will also help the . In order to support an heir in Poland the player will need a royal marriage with them. If starting far from Poland, you may need to improve relations with them before they will accept an offer for a royal marriage. Improving relation will also help raise Poland's opinion of the player's nation, which as mentioned, will increase the support heir chance. Having secured a royal marriage with Poland, allow time to pass until they get the Successor of Wladyslaw III event. If Poland's interregum ends before this event triggers, they will not be able to become an elective monarchy, and so starting again will be required. Assuming Poland do get the event they will usually select the option to form the union over 立陶宛 and become and elective monarchy. However, if they do not select this option, then once again, they will not be able to become an elective monarchy and the player will need to restart and try again. Assuming that Poland does get the event, and accepts the option leading to them becoming an elective monarchy, the player should send a diplomat to Poland to undertake the support heir action listed under the Dynastic actions sub-menu. From this point it is a matter of good luck relating to both the successes in accruing heir support and waiting for Poland's current ruler to die. If their ruler dies while your heir has the greatest support of all heirs, then they will assume the throne. As long as the player's nation's dynasty has not changed since beginning to support the heir, then this will mean that a ruler of the same dynasty as the player's will be on the Polish thone and the achievement conditions will have been fulfilled.