Vegeta actually did pretty well against #18 at first. She only gained the clear advantage when his ki depleted and hers [obviously] did not... And then Trunks went on to say that these Androids were far stronger than the ones from his timeline.
Iirc, Trunks even stated that he was able to hold his own against the Androids in his timeline for a bit, yet #18 in the mainstream timeline dropped him with a shot or two. So the difference in power was evidently quite large.
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"I am tired of Earth. These people.
I am tired of being caught in the tangle of their lives."
Gohan did well against his Androids, but the mainline Androids are way stronger and Vegeta fought on equal footing with 18.
Vegeta was actually pretty ****ing strong for an early SSJ1. Take into account both Gohan and Trunks had years of experience as SSJ1's when they fought weaker Androids and got somewhat stomped whereas Vegeta had to have had it at like... what? A week or two the latest when he fought 18? Though maybe I'm remembering it wrong.
Hell, as of "History of Trunks", Gohan had been a Super Saiyan for over 10 ****ing years, IIRC.