OK. Here's what you need to do:
1) Create this formula and place this on the Group Footer #1:
EvaluateAfter({@Formula_Name_in_Step2});
numbervar cnt;
(DistinctCount({PO}, {Vendor_Group_Field}) / cnt) * 100;
Note: Replace {@Formula_Name_in_Step2} with the name of the formula you've created in Point 2 above.
2) Create another formula and place this on the Report Footer:
WhilePrintingRecords;
numberbar cnt2;
(DistinctCount({PO}) / cnt2) * 100;
-Abhilash