LEFT JOIN Producing Inflated COUNT() numbers
If you run the query without aggregates, you'll see what is happening:

Select
    *
FROM @AllVendor av
    LEFT JOIN @BBC bbc
        ON av.vendor = bbc.vendor
    LEFT JOIN @TotalSalesAmt tsa
        ON tsa.vendor = av.vendor
where av.vendor = 'vendor 1'

+----------+----------+---------------+---------------------+----------+---------+
| vendor   |  vendor  | vendorcasenum |     casenumdate     |   vendor | saleamt |
+----------+----------+---------------+---------------------+----------+---------+
| Vendor 1 | Vendor 1 | E12301        | 06.11.2013 00:00:00 | Vendor 1 | 10,20   |
| Vendor 1 | Vendor 1 | E12301        | 06.11.2013 00:00:00 | Vendor 1 | 0,40    |
| Vendor 1 | Vendor 1 | E12301        | 06.11.2013 00:00:00 | Vendor 1 | 20,10   |
| Vendor 1 | Vendor 1 | E12221        | 06.11.2013 00:00:00 | Vendor 1 | 10,20   |
| Vendor 1 | Vendor 1 | E12221        | 06.11.2013 00:00:00 | Vendor 1 | 0,40    |
| Vendor 1 | Vendor 1 | E12221        | 06.11.2013 00:00:00 | Vendor 1 | 20,10   |
+----------+----------+---------------+---------------------+----------+---------+


Due there are 2 rows in @BBC table of Vendor 1:

('Vendor 1',   'E12301',    '2013-11-06')
('Vendor 1',   'E12221',    '2013-11-06')


The aggregated SUM(saleamt) = 61.40

You can use one subquery to calculate SUM(saleamt) and another to calculate COUNT(vendorcasenum):

Select
    vendor,
    (select ISNULL(SUM(tsa.saleamt),0)
     from @TotalSalesAmt tsa
     where tsa.vendor = av.vendor) TotalSales,
    (select COUNT(bbc.vendorcasenum)
     from @BBC bbc
     where av.vendor = bbc.vendor) TotalCases
FROM @AllVendor av
ORDER BY av.vendor ASC;


This is the final result:

+-----------+------------+------------+
|   vendor  | TotalSales | TotalCases |
+-----------+------------+------------+
| Vendor 1  | 30,70      | 2          |
| Vendor 10 | 0,00       | 1          |
| Vendor 11 | 0,00       | 1          |
| Vendor 12 | 0,00       | 0          |
| Vendor 13 | 0,00       | 0          |
| Vendor 14 | 0,00       | 0          |
| Vendor 15 | 0,00       | 0          |
| Vendor 16 | 0,00       | 0          |
| Vendor 17 | 0,00       | 0          |
| Vendor 18 | 0,00       | 0          |
| Vendor 19 | 0,00       | 0          |
| Vendor 2  | 30,20      | 1          |
| Vendor 20 | 0,00       | 0          |
| Vendor 3  | 20,00      | 1          |
| Vendor 4  | 0,00       | 1          |
| Vendor 5  | 0,00       | 1          |
| Vendor 6  | 0,00       | 1          |
| Vendor 7  | 0,00       | 1          |
| Vendor 8  | 0,00       | 1          |
| Vendor 9  | 0,00       | 1          |
+-----------+------------+------------+