I would like to purchase an extra licence for my site, please help me

I would like to purchase one more licence for my site,
however, I can't see how it will make any difference as I have

http://www.123.com/vb
and
http://www.123.net/vb

both those are mine are in the same folder with the same content
I would like to keep the .com links to keep my google rank
but also I would like to get .net version available.
can you help please?

I would discourage you from this as it will be a ton of duplicate content. Also, vBSEO will only work with the URL that is defined in the admincp, so you need to do some external code modifications.