Personally I think it's just impossible to model a Dutch vassal spawn without historical railroading. The fact that Spain came across possession of the Netherlands was through Austrian royal marriages that produced the Burgundian inheritance, and in turn Spain acquired that portion of the Habsburg pie due to splitting of the Habsburg territories following Charles V. You can't model any of these things organically in game in a way that would not just railroad it to history. Further, the history itself is subject to very easy revision: what if Ferdinand got the Netherlands rather than Philip? So if you decided to railroad the Habsburg inheritance, you could have the situation resolve either with a Spain or HRE master. But then even further back, what if Mary of Burgundy married an English prince, or a French prince, or a Byzantine prince, or a Russian prince? The inheritance would then devolve to that country and the Dutch should spawn as their vassal, but how in the world could you even get close to modelling that? Then for gameplay concerns, you don't have the same situation with Canada (or any other Commonwealth country that's added) where they operated pseudo-independent until 1931 and the Statute of Westminster. The Dutch were not autonomous at all within the Spanish Empire and won independence through force of arms rather than gradual peaceful devolution. This outcome is better modelled with a sudden flip than with a gradual slow grind. Having the Dutch just spawn independently and presume they won independence from whoever is the best outcome for gameplay and history.