Id like to keep this post open if people are interested....
I found the code you mentioned (seagul)
Search "$vendor_id = 1" (2 hits in 2 files)
administrator\components\com_virtuemart\plugins\vmpsplugin.php (1 hits) Line 859: $vendor_id = 1;
administrator\components\com_virtuemart\views\shipmentmethod\view.html.php (1 hits) Line 59: $vendor_id = 1;
I also found another instance
administrator\components\com_virtuemart\elements\vmcurrencies.php (1 hits) Line 48: $vendor_id = VirtueMartModelVendor::getLoggedVendor();
This is more probable cause;
administrator\components\com_virtuemart\models\user.php
Line 575: //TODO Attention this is set now to virtuemart_vendor_id=1, because using a vendor with different id then 1 is not completly supported and can lead to bugs
Line 576: //So we disable the possibility to store vendors not with virtuemart_vendor_id = 1
Other info found
+ Search "virtuemart_vendor_id" (347 hits in 93 files)
+ Search "['virtuemart_vendor_id'] = 1" (4 hits in 3 files)
+ Search "`virtuemart_vendor_id` = "1"" (4 hits in 4 files)
+ Search "virtuemart_vendor_id = 1" (7 hits in 5 files)
Whilst I would love to solve this it goes beyond my abilities.
Im happy to help with bugs but perhaps someone else could look at it from here on?
A couple of weeks work could make this the vendor cart we have all been searching for....
DM