The four children of Wladyslaw IV and Jan II of Poland, died in infancy.
Wladyslaw IV by Cecilia Renata of Austria
Zygmunt Kazimierz (1640-1647) died of dysentery
Maria Anna Izabela (1642-1642) died of a convulsions.
A daughter (1644), died at birth.
Jan II Kazimierz by Ludwika Maria of Gonzaga-Nevers
Maria Anna Teresa (1650-1651), died of unknown causes
Karol Ludwik/Jan Zygmunt (1652-1652) died of unknown causes
Since I can't seem to find the causes of death for Jan II Kazimierz' children, let's assume they died of something equally easily butterfly-able like their cousins.
If there were to be a new generation of Vasa royalty for Poland, would it make any difference in the long run to the future of the PLC? And just as an added butterfly, with a Polish-born prince favorable to a foreigner, the king (whichever one could actually manage it) gets the Sejm to institute the vivente rege to prevent some foreign/local (non-royal) prince from seizing power during a regency.