Alright, so the following calculates sales for a particular month irrespective of the Lender:
1) Create a formula for each month. The code for Jan would be:
If Month({date_field}) = 1 then {field to sum}
2) Inserting a sum on this formula field will give you total sales for Jan irrespective of the Lender.
3) You can then use this summary in another formula calculate the percentage of sales by Vendor for a month.
Could you post a screenshot of how you want the report to look-like (with all the monthly sums) and how it looks now?
-Abhilash