Hi,
"Sorting/ordering" (manufacturer, category, product etc..) are all taken care by Virtuemart Core. To change the ordering; custom coding modifications on Virtuemart core files is required; which is beyond our scope. There is a configuration for "products" in the backend to change the product-sorting easily; but not for others. You should raise this issue in the Virtuemart Forum and ask them to add "configuration" for sorting for manufacturer in VM Backend as well (but it is a very tiny detail; and i won't think they will ever think about adding this configuration to VM backend)
In template; we "display" manufacturers in the dropdown in the "same" order Virtuemart Core provides to front-end.
FYI: Manufacturers in the dropdown is ordered by ManufacturerID (descending) by using this method: orderByList (search this method in the VM core files and you will find it; actually the real sorting-foreach-listing is handled via this PHP file: ROOT//administrator/components/com_virtuemart/models/product.php -- so you have to hack this file to change the default ordering for manufacturer in the dropdown -- DON'T TOUCH THIS FILE UNLESS YOU ARE AN EXPERT ON PHP CODING !!!
Regards.