You can set up a command to do this.
In your Database expert there is the Add Command option just above your DB source.
Double click the Add command and put the following code
SELECT FIELD (your vendor field that you want to select from here)
FROM table (the table your vendor field is in)
UNION
SELECT '...All'
In the Database Expert, Join this new command on the vendorname field to the vendor table on the vendor name field.
Point your Paramenter field to this command field instead of your vendor.vendorname.
Change your select expert to handle this with an if statement.
if {?Paramter}="...All" then
{Vendor.vendorname}={Command.field}
else {?Parameter}={Command.field}