I'm pretty shure that the following will solve your problem:
Look at your database table with phpMyAdmin and search for the tables with a language suffix at the end. I suppose you orginally installed VM2 in english. Therefore you should find the following tables:
m0atm_virtuemart_categories_en_gb
m0atm_virtuemart_manufacturercategories_en_gb
m0atm_virtuemart_manufacturers_en_gb
m0atm_virtuemart_paymentmethods_en_gb
m0atm_virtuemart_products_en_gb
m0atm_virtuemart_shipmentmethods_en_gb
m0atm_virtuemart_vendors_en_gb
If one or more of these tables is missing with the sl_si suffix at the end, export the english table, rename the exported file suffix to sl_si and import it again. When you are done, you should also have the sl_si table versions in your database.
m0atm_virtuemart_categories_sl_si
m0atm_virtuemart_manufacturercategories_sl_si
m0atm_virtuemart_manufacturers_sl_si
m0atm_virtuemart_paymentmethods_sl_si
m0atm_virtuemart_products_sl_si
m0atm_virtuemart_shipmentmethods_sl_si
m0atm_virtuemart_vendors_sl_si