I don't think it's too sad--I mean, she and Zack are still on good terms.

I do agree, I was really happy to see how Cloud and Vincent interacted in AC--those two have always had very strong parallels, and the creators made sure to emphasize this in Advent Children. The scene that always gets me though is Marlene hiding in Vincent's cape. XD

Good luck with Crisis Core--you're going to see a bunch of highly familiar scenes. One good thing I do note in its favor is that Zack is hilarious. Srsly--summon cactuar, you'll see what I mean. XD