In my experience, you don't need the diplo slots as you can steamroll basically anyone. You would still have enough to get a large ally (or more if you go for Influence/Diplomatic). The thing I generally don't enjoy about making Gascony a vassal is that Vassals count for each Vassal you own when it compares relative power for LD, whereas PU's count individually. I tend to vassalize other minors and I don't want to have a ton of vassals with increasing liberty desire. Hence why I feed Navarra lots of France. It also helps give Navarra a purpose instead of being that taken up OPM diplo slot for 50 years. The main thing Aragon has over Castile is (if the Wedding is timed right) you get tons of colonies made in your culture group for free. You don't need to do anything while you sit back and have Castile make them for you.