As you've no doubt discovered, Monarch doesn't handle multiple key Top N summaries well at all. Single keys only please. Unfortunately, that doesn't satisfy requirements such as what you've described.
But fear not, you can get what you want with Monarch, but it takes more initial work than you'd guess. The good news is that since we're talking about Monarch, it's still only a few minutes of effort.
Here's my technique for such work.
Start with your summary, but don't worry about the Top N limits. Just build your summary with Location, Vendor and SUM(Sales). Do not include subtotals or a grand total. Export that out to a file.
New model: Import that export and add an Original_RecNo calculated field using RecNo(). Export that to a new file.
New model: Import the first export and using an external lookup to the Original RecNo export, import the Original Recno field based on the Location field. This will give you the record number of the first occurrence of a particular location for every record for that location.
Now add a Top 15 filter:
You can make it easier to maintain and more flexible if you make the value of 15 a runtime parameter. You'll have an instant Top 3, Top 10, whatever.
Ideally if you're doing this sort of work regularly, use a batch file and project files to make your life easier.
I hope that I've described this properly and haven't overlooked any steps.