I need to generate an invoice for a product and send to an already registered client who will then come to the site and pay the invoice -is tihs how this product works?

Hello,

the invoices are being generated based on VirtueMart orders. So, if the order does exists or you will create it (or your client will do so), then VM Invoice can generate the PDF invoice that may be sent back to client.

What you cannot do at the moment is to create "new" order in VirtueMart using VM Invoice. So first the order must exists, then you can generate invoices.