Why do people disregard Vegeta's effort against Super Perfect Cell?

Who disregards it and doesn't think it's a big point of the fight? I mean it's obvious. In fact...that's why it was in the manga/anime.

The theory about Gohan is well...Gohan was tired from fighting and on top of that one of his arms were destroyed. That's where the, "He was at half power" theory comes in and I actually subscribe to it. I believe that Gohan's rage blast after Vegeta's distraction is a burst and he wouldn't be able to sustain it. That's why it was important for Cell to keep his eye off the ball. If Vegeta didn't distract Cell and Gohan just let loose that power, I think Cell's beam would be overtaken, but not for long enough. Or Cell could have a burst of his own. In any event, I don't know why people think that Cell was more powerful than Gohan at that point. Gohan beat him (distraction, no distraction, whatever) with a pretty big handicap. Had he had both arms to perform the Kamehame-Ha with, I don't think he'd be quite so worried about losing the struggle.

But anyway to your point...I want to see one person argue that Vegeta's distraction didn't do anything.

I agree with Gohan's blast at the end being a burst. It's happened, before I think - I'm pretty sure Goku did something similar for a moment when he and Frieza were clashing (Super Kamehameha vs. Nova Strike), causing Frieza to move. Anyway, bursts last momentarily so that's why Gohan needed to find the right time. I believe Gohan was already using his "full power" (while weakened), but he just needed an extra "push" if that makes any sense. The only problem would be that Cell could easily do the same thing, and Cell had the advantage as it already was. So, Vegeta's Big Bang was a turning point - it almost visibly knocked Cell over, if I remember correctly, giving Gohan the chance he needed. I don't think it's disregarded, though - I think what people say is that Gohan would've won more easily had he been at full power, in which case, the Big Bang Attack (arguably) wouldn't have made much difference (unless you argue that Cell never got a chance to unleash his own true power - I've seen some who believe this).

Vegeta wasn't the only one at fault there.

Krillin should have grown a pair and just disabled Android #18. How can you fall in love with an android after one kiss on the cheek? Much as I like #18, the world would've been saved a lot of trouble if she was destroyed. And she didn't do anything later on that made her existence necessary.

Trunks could have destroyed Android #18 the second he noticed her location, yet instead chose to let her be and go after Cell.

Piccolo could have destroyed Cell if he hadn't been such a damn fool and hadn't fallen for the Solar Flare. Honestly, you'd think they'd be prepared for this move by now.

Android #16 had ample opportunity to destroy Cell as well, but I think he was overconfident because he gave Cell way too much time.

So you see, Cell's completion is not only to blame on Vegeta. Although his mistake was definitely the biggest one.

Most of what y'all are saying is correct, but you're missing the most crucial point - all villains have special plot armor on 'em until they reach their final/true/most powerful form (or just reach full power at all).
But true, if not for Krillin and particularly Vegeta, there'd be no Perfect Cell. As Piccolo even said, Vegeta could've just killed Cell but didn't (on second thought, even Piccolo could've killed Cell, but if I recall, he spent too much time going "I'm not scared of you", "You still have no idea what you're dealing with!" Ah, but Solar Flare.....)

Well I don't really care which one is more at fault or to blame... Everyone made a dumb decision, whether it was a honest mistake, an emotion of love, or exaggerated pride... Each one of them had a chance to stop Cell, and each one of them let it slip through their fingers. Cell becoming complete is on all their heads, not just Vegeta's.

Vegeta didn't even know Krillen was there. I am not saying Krillen didn't mess up, because he sure as hell did. What I am saying is the that the reason Vegeta let Cell go was out of selfishness, and his stupid pride....There is a huge difference there...Then he goes and pulls the same shit in the Boo saga, and then has the audacity to tell Goku that the humans should help themselves every once in a while...Dumbass!!! nobody would be in this mess if not for you.