Forwarding means that any emails sent to, say your old virgin.net address will be received and then sent on to your new address. But once VM close down your old address, this will no longer happen, the old address will cease to exist and any mail sent to it will simply bounce back to the sender with a message along the lines of non-existant addresss.
Wht you need to do is to contact all of your existing, well, contacts, and inform them of your new address and to only use that in future. You then need to export or save all of your existing emails from the VM account (assume that it will all vaporise after you leave), and then import them into your new Outlook account - there are numerous ways to do this.
In answer to your question, anything that gets forwarded to your new account, yes you can open attachments etc. When and if VM delete your old account, then all of those messages will be irretrievably lost, unless you have exported them first.
