I don't know how you migrated your Joomla 1.0.x to J1.5, but for myself, the way I migrated is by installing a completely new clean J1.5 installation and all the necessary components. Then I did the database manipulation. That way will give me no error for every of my installed extensions.
Make sure you upgraded your Joomla correctly. Because by default, VM 1.1.2 can handle more than one payment method easily. For related items, I just tried to modify one of my products, and it works fine either.
My suggestion for you, try to remove the payment method and readd. Also try to remove related items of one of your products and readd.