The issue is that the "internal" field value calculations in VM invoice do not follow the logic of the invoice. This way you'll get different results and the invoice amounts do not add up.
However the root cause which brings the poor coding of the logic to the surface, is that VM by default calculates with 5 digits after the decimal point - for whatever reason. Once this is changed to 2 digits, the issue disappears.
So go into phpMyAdmin and change the field types in the STRUCTURE of the following tables:
##_vm_product_prices
product_price decimal(15,5) ->decimal(15,2)
product_override_price decimal(15,5) ->decimal(15,2)
##_vm_order_items
product_item_price decimal(15,5) -> decimal(15,2)
product_tax decimal(15,5) -> decimal(15,2)
product_basePriceWithTax decimal(15,5) -> decimal(15,2)
product_finalPrice decimal(15,5) -> decimal(15,2)
product_subtotal_discount decimal(15,5) -> decimal(15,2)
product_subtotal_with_tax decimal(15,5) -> decimal(15,2)
##_vm_orders
order_total decimal(15,5) -> decimal(15,2)
order_salesPrice decimal(15,5) -> decimal(15,2)
order_billTaxAmount decimal(15,5) -> decimal(15,2)
order_discountAmount decimal(15,5) -> decimal(15,2)
order_subtotal decimal(15,5) -> decimal(15,2)
order_tax decimal(15,5) -> decimal(15,2)
order_shipment decimal(15,5) -> decimal(15,2)
order_shipment_tax decimal(15,5) -> decimal(15,2)
order_payment decimal(15,5) -> decimal(15,2)
order_payment_tax decimal(15,5) -> decimal(15,2)
As my closing words: I am very disappointed about the support for these commercial addons. After three weeks of non-response I ended up digging into their coding logic myself, because the calculations in their invoices do not add up. This is not very professional..... :(