With trips that leave domicile and don't return you just have to cover a day of the trip to have the whole thing dropped / pay credited.

Yes we can maximize vacation. The days of vacation vs credit hours vacation cuts both ways though. If you're VERY junior you could end up having the majority of your vacation fall on days scheduled off already - we bid pre-built lines, not PBS. For the vast majority, the day system works great and respects seniority absolutely. If you're #1 you can bid a line where a 7 day vacation period touches two 26hr 5 day trips, and have them both dropped/paid. If you're on reserve you get paid 4hrs per day of reserve your vacation covers.

We can slide vacation, but it get complicated quickly. Half the pilots here don't understand it ;-)