Cron Every 2 Days
 

Is it possible to make a cron run every two days instead of every day?

Not easily, no.

In unix */2 in the "day" field runs an event every other day, though that isn't available in the vBulletin cron.

The cron job itself could be run daily, and have the code itself test for even number in the date, but it would trip up on months with 31 days, leap years, etc.

vBulletin Scheduled Tasks are not the same as a standard cronjob. Scheduled Tasks are not "precise" and are only run when users (and guests) land on a vBulletin page.