When I add
define('MULTISITES_ID','metamedicine_com');

To the joomla site config in the slave, then the add button redirects correctly to metamedicine.com
However, then the images are not shown.

Please advise, as I am getting frustrated with this component

The define MULTISITES is automatically compute by JMS when you are displaying the correct websites. So if you see the correct content, this means that it is dedined.
So you don't have to add it twice.

If the JMS patch for the VM Configuration wrapper is correctly installed in the master, you should read the correct VM config file.

So if seems that you have patches not installed or I don't know the reason.

MASTER site: All patches are installed like you can see below.
Joomla Multi Sites version: 1.2.10 (Latest available: 1.2.10)
Patches definition version: 1.2.13 (Latest available: 1.2.13)
Files Status
administrator/components/com_multisites/install.xml
OK
administrator/index.php
OK
administrator/includes/defines.php
OK
includes/defines.php
OK
includes/defines_multisites.php
OK
includes/multisites.php
OK
installation
OK
installation/includes/defines.php
OK
installation/installer/helper.php
OK
installation/installer/jajax.php
OK
xmlrpc/includes/defines.php
OK
configuration.php
OK
administrator/components/com_config/controllers/application.php
OK
administrator/components/com_installer/models/templates.php
OK
administrator/components/com_modules/models/module.php
OK
administrator/components/com_templates/admin.templates.html.php
OK
administrator/components/com_templates/controller.php
OK
libraries/joomla/application/application.php
OK
libraries/joomla/document/html/html.php
OK
libraries/joomla/filesystem/folder.php
OK
libraries/joomla/session/session.php
OK
components/com_content/helpers/route.php
OK
plugins/system/remember.php
OK
administrator/components/com_jce/installer/installer.php
OK
administrator/components/com_virtuemart/classes/ps_checkout.php
OK
administrator/components/com_virtuemart/classes/ps_config.php
OK
administrator/components/com_virtuemart/classes/ps_order.php
OK
administrator/components/com_virtuemart/virtuemart.cfg.php
OK

Please let me know :
1) where the multisites_id is defined by JMS
2) where in VM the connfiguration of the frontend is retrieved

Then I will look into the code to see what happens, and let you know when it is solved.

p.s. I even downloaded the newest version and installed that... all went fine... no errors whatsoever

The Multisites ID is computed at the begining of the processing before Joomla start.

Normally, if the VM virtuemart.cfg.php is called directly (that normally is not the case), it contain a wrapper that allow to call JMS to compute the multisites id.

This is a complex algorithm that I don't want to develop here.

In summary, if the correct website is displayed, this means that multisites id is defined because this is it that trigger the dispaly of the correct webpage.

If your add button has not the correct URL then I don't know the reason.

You have ordered JMS on www.jms2win.com that use VirtueMart and you had any problem with the URL. It uses exactly the same code.
So the problem is somewhere else and I don't know where.
The only solution is to debug your specific environment.

Based on your trace, it seems that the administrator/components/com_virtuemart/virtuemart.cfg.php is correctly installed. So I don't know why.

Perhaps you are using a specific themes (templates) that would bypass everything.
I don't know.

Even with the default template for Joomla and VM the component does not work
I already spend way too much time to get this component working the way I want to.

I revert back to separate installs of joomla and VM

Please refund my money and I will delete your component