Some times people just notice it, kinda like how you pass by a tree and out of the corner of your eye you see a cicada on the tree so you do a double take and see that's actually there.
I also think it has something to do with pattern recognition, you see a pattern so it's easier to spot when that pattern is disrupted and vice versa.

To be fair, Vegeta saves Goku's life afew times.

*In the entire series. (I'm probably over exaggerating)

Actually Vegeta was really useful. One exampe is during the clash between Cell and SSJ2 Gohan, Vegeta is the only one with enough power to successfully distract Cell which lets Gohan finish him. Another is during the Majinn Buu saga when Vegeta stalls Kid Buu, someone immensely more powerful than him, while Goku regains his energy. A third example is during the Android series when Goku's heart disease lets Android 19 nearly kill Goku. If Vegeta did not come, everyone would have died right there. There are more, but that's what just popped into my head.

I just mean he never actually kills any of the main antagonists, it's my main gripe with the series the character has a lot of wasted potential. Two main points that show this potential is in the cell and buu sagas, when (spoilers) Trunks is shot by Cell's Death Beam and presumed dead Vegeta realises what a cunt he's been and rages at cell and that results in nothing he doesn't even go SSJ2 - which is retarded (but its probably due to his power level)- and when he realises he needs to fight for his family and that he loves them he fails as well in the Buu Saga Its the exact same problem Gohan and Piccolo have: strong characters that get outclassed by ridiculous power. Vegeta and Piccolo are still my favourites though.

If you look at Vegeta as a character, he is the one always striving to be better than everyone else but never reaches that level of power. Goku is also the main protagonist, and it is expected that he would finish the major battles (excluding Cell). That's why Vegeta fails at killing the main antagonist all the time. Although, Vegeta has killed some of the minor antagonists in their respective sagas. If you look at the Cell saga again, Vegeta does become the second most powerful character (next to trunks), and could have easily killed Cell. That chance was lost due to his arrogance though.

mfw Vegeta is my favorite character of all of DBZ

Thank you sir :D