From the active acetone fraction of Chenopodium murale, vanillic acid was isolated and identified, based on $^1H$ and $^{13}C$ nuclear magnetic resonance spectral analyses. Free phenolic compounds inside the active acetone were qualified and quantified by using high performance liquid chromatography analysis, which revealed the presence of seven compounds with an abundance of vanillic and p-hydroxybenzoic acids. The allelopathic potential of the acetone fraction and vanillic acid was evaluated through laboratory bioassays against tomato plants. Our results showed that the allelopathic potential induced by low concentrations of the acetone fraction and vanillic acid stimulated the germination and growth of tomato and had stimulating effects on the activity of some antioxidant enzymes. We observed an enhancement in the activity of catalase, peroxidase, superoxide dismutase, and polyphenol oxidase, as well as the content of soluble protein and phenolic glycoside. Meanwhile, the levels of free phenolic compounds, $H_2O_2$, and lipid peroxidation decreased. The highest stimulations were recorded at 50 p.p.m. of acetone fraction and 0.5 p.p.m. of vanillic acid. In contrast, the highest concentrations exerted negative effects on all the measured parameters to record the maximum value of inhibition at 400 p.p.m. of acetone fraction and 4 p.p.m. of vanillic acid. These results proved the antioxidative effects of active acetone and vanillic acid at low concentrations and their potent use as a stimulator for tomato germination and growth.