THe veg oil is for ang eggs the snake might have attatched to it. It seals them and doesnt allow the eggs to hatch. and if there are any left in the tank, it doesne allow them to get a hold and cant pester the snake. And also suffocates the extra mites. Bugs breath through their skin so thus the oil wont let the little buggies breath.
ALSO DONT FORGET TO VACCUM THE WHOLE ROOM AROUND YOUR SNAKE TANK The mites can travel 20 feet a day.
Regards,
Beauty the BP