Butter is 85% fat and 15% water (approximately). A fair substitution would be to substitute 8T of oil and 1T of water for 9T of butter. Going the other way, use 9T of butter instead of 8T of oil, and reduce other water in the recipe by 1T.

However, butterfat is a solid fat, while vegetable oil is liquid. If your recipe depends on the solidness of the fat, like most pastry recipes do, the recipe will fail if you substitute oil for butter.