How did the Kaioken X4 not obliterate Vegeta?

[caejones]

It's almost like flat numerical comparisons aren't the be-all and end-all of fighting.

I think I love you.
Piccolo says effectively the same thing to #20, having demonstrated it time and time again before then.

But if we must get mathy, think about stuff like body composition (Saibamen look like cabbages, and I recall a scene from a non-superpowered anime in which someone unintentionally exploded a cabbage by trying to kick it). Meanwhile, humans, especially well-trained ones, especially the super-well-trained ones like Saiyans like Vegeta, can take considerably more than a cabbage. (Though, I suppose by this token we should have expected Saibamen to lose bodyparts while fighting...)
Then there's pressure. How ki maps to real-world physics is merky at best, but irl we use blades and drills instead of hammers for a reason. I'd call the kamehameha more of a hammer than a blade. Now, let's try a sledgehammer roughly you-sized, partly slowed by your own sledgehammer that it just knocked out of your hands, with you being able to put up some kinda defense. Even if this thing comes at you with way more power than you can put against it, you're going flying. But what else?
Or better yet. Get hit by a bus that had to bust through a stack of bricks to get at you, while you are braced for the impact and know how to avoid a bone-shattering landing.
Given the choice between a buzzsaw through the chest and the bus, which would you expect to survive?
Now, I'm not about to go throw myself in front of a bus to test this (that's what we have Mythbusters for), but... that's an anlogy, I guess?

In Cell's case, set the bus on fire, and make it bigger, and take away the bracing-for-disaster.

[Xyex]

I think Vegeta's Galick Gun weakened the blast some.

Then how was Gohan able to obliterate Cell? Shouldn't Cell's Kamehameha have softened the blast?

Cell was engulfed, which (IMO at least) makes all the difference. Only the 'head' of the attack is likely to lose much in the way of energy, the stream that follows will still have full force to it. Beyond that, the Cell/Gohan power gap is most likely quite a bit larger, and Cell was also distracted before being hit.

[FNF]

Wouldn't 'because AT didn't want to kill him off' suffice? I mean, talk about anti-climatic endings...

[dprez]

Vegeta's durability and the bulk of the Kamehameha's power being used to counter Vegeta's blast. Because of this, I think that by the time it hits Vegeta, it could've been a much weaker blast, allowing him to roll off it before it caused further damage. Every ounce of Vegetas power was being used to try to push the kamehameha back. When it finally directly hits him, he could've weakened it enough so it wouldn't obliterate him on contact.

[Bando]

I think Vegeta's Galick Gun weakened the blast some.

Then how was Gohan able to obliterate Cell? Shouldn't Cell's Kamehameha have softened the blast?

Gohan kept firing energy into the Kamehameha whilst Goku only let out a strong burst then stopped.

[SSJ4_Zankuto]

Goku did say to Yajirobi afterwards that Vegeta will recover from the Kaio-Ken x4 Kamehame-Ha! I'd say Goku held back some of the power in the Kaio-Ken x4 Kamehame-Ha! to just enough to push back the Gyallic-Ho and preventing Vegeta from being destroyed from it.

[Piccolo Daimao]

Goku did say to Yajirobi afterwards that Vegeta will recover from the Kaio-Ken x4 Kamehame-Ha! I'd say Goku held back some of the power in the Kaio-Ken x4 Kamehame-Ha! to just enough to push back the Gyallic-Ho and preventing Vegeta from being destroyed from it.

I don't know where you got that implication from. All he said was, "If he was killed that easy, then I wouldn't have had so much trouble." I don't think Gokuu intentionally held back using what was such a dangerous technique for himself anyway, because it was a matter of life-and-death.