This is largely because Vegeta is coming off the high of demolishing Android 19 (another satisfying Androids Saga fight) and feels invincible, only to quickly be humbled by Android 18's superior ...

The Android saga opens up with Dr. Gero and Android 19 terrorizing the population, and while they prove to be too strong for other Z-fighters, Vegeta easily takes on Android 19 and kills it.