Yes and no.
Yes this can be done. No, it's not just an If function. As long as you have a Vendor ID field in your report table, you can query that table for the same vendor ID, same project as that of the current report record but with a date less than the date of the current report record. Sort those records by date and you can find the most recent prior report In order to compare dates.
You can use any of the following;
1) A sorted self join on your report table
3) a scripted Find and sort.