When you say vegetable oil, do you meant soybean? Corn? Peanut? Is it a big name brand?
Every type of oil has a different level of saturation. The more saturated an oil is, the less susceptible it is to spoiling. The extreme end of this is crisco (hydrogenated oil). Hydrogenation is basically artificial saturation.
Impurities are also a factor in oil shelf life. A heavily processed bottle of wesson soybean oil will last quite a bit longer than an impurity rich oil like extra virgin olive. The more refined an oil is, the longer it lasts.
Is it a dark cupboard? Is it in plastic? Glass? Clear or dark? Is the cupboard warm? Has the bottle been opened?
Heat, light and air are all enemies of oil.
There's an unknown factor here as well. What kind of treatment did your oil receive at the bottling plant or the supermarket? Did they leave the palette out on a sunny loading dock in the middle of summer for a few days?
I buy small bottles of expeller pressed oil packed with nitrogen, I add a gel cap of vitamin e to each and I store it in the refrigerator. But then I take fresh oil really seriously.