Treated wood should have no issues with touching the ground. I would
put a vapor barrier between the dirt and the living area though. The
reason for vents is to dry out the space, but the humidity in the
space is the same as the outside air and moisture constantly comes
through the ground. Vents are optional, but neccessary if you do not
place a vapor barrier on the soil. I would dig down far enough to
retard any creatures from going under your "walls". Build a perimeter
with treated wood. Insulate the inside of the perimeter with some
solid foam insulation and then lay down a vapor barrier on the inside
soil and up the inside perimeter walls. Make sure to "seal" (silicone)
the joint between the barrier and where it comes to the living space
joists. Just my opinion. Good luck.