What format for your report do you need?
A list of all PO's grouped by Vendor with subtotals for each vendor? for each Month? For each vendor for each month?
Or do you want a report that lists each item purchased and has similar subtotals?
In either case you won't have formulas as such but instead you use summary fields that compute totals of fields in the same table. Then it's a matter of seeting up sub summary layout parts and putting the same summary field in different sub summary layout parts to compute your monthly and other sub-totals--then sorting your records by the same "when sorted by" fields specified in your sub summary parts.
Here's a tutorial on summary reports: Creating Filemaker Pro summary reports--Tutorial