"One of the last" - they all started dying pretty late into the game and you fought Vexen once before, Larxene once before, Axel as well, and Repliku a couple of times. Makes sense it'd seem like he was 'one of the last' to die in light of the repeating boss battles.
And no, see, that's the thing - it took 4 days to reach there.
The Org Members left for CO on the 20th something Day. That leaves them less than a WEEK to get to Vexen's death - and then 20-30 days of Sora going through the two and a half floors he had to go through to get to the pod, supposedly on day 49. And that's what bugs people - that the majority of Sora's story took less than a week, but the minority of the castle took almost a month.
I can understand them wanting Roxas to be passed out for the duration of CoM, and still see Xion's face and see her wield the Keyblade (which add up with Sora all but forgetting Kairi by then so however it might've affected Xion - it's all good), but because of that, they killed Vexen far too early into _Days_ to make it work.
If they had put Vexen's death on, iono, even Day 36, it would've added up better, as the top floors are the bigger ones so you could somehow play with that. But to kill him on Day 26 and then have Sora's part of the story extend for 20 more days? illogical much?