The standard module will only allow you to specify one Category from which it selects a list of Products.
This is a really simple hack to change the SQL to allow a comma separated list in the category ID field.
Change lines as follows:
33: $category_id = $params->get( 'category_id', 2 ); // Display products from these categories only
51: $q .= "AND #__{vm}_category.category_id in ($category_id)";
or see attached file which should replace the installed file in your-site/modules/mod_virtuemart_randomprod
NOTE: remove the ".txt" from the end of the file name
[attachment cleanup by admin]