Almost all the VirtueMart payment method save their configuration in files on the disk.
So perhaps that your "Sage Pay" payment method is also doing that.
So if you want a "Multisites Sage Pay" method for VirtueMart, you should consider modifying the current payment gateway.
For all the standard virtuemart payment method, we experiment a new product that will patch most of the virtuemart payment method.
We hope to release this new product soon. This would give the possibility to have different paypal email account or merchant account for the standard VirtueMart payment method. IT is not yet release because we still not have written the documentation.
If you want that we create a specifci "multisites save pay" payment method, this is possible with our billable support that you can order at
www.jms2win.com/en/joomla-multisite?page...35&category_id=1