First off, the upgrade/renewal fee is $40.
Since you purchased Photopost, has acquired vbgallery, and currently maintains both products.
While all their products integrate nicely, vbgallery is a little more integrated into vbgallery. It's not a huge bonus, other than the settings are integrated in the vbulletin admincp.
I believe vbgallery is up to version 2.4.3, and photopost is around 6.2.
Photopost can be a stand alone version. However it can be integrated into vbulletin.
With Vbgallery, you MUST have vbulletin. This product is not a stand alone system.
For upgrading your photopost license, go to the member's area (link at the top of the page), and get the license number of the product you wish to upgrade. Save this for later. Now you will see a link to renew the license.
You walk through the steps, and during the process there should be a comment block. This is where you tell them your license in which you are upgrading.
Once you complete the transaction, within 24 hours you should get an email saying it's comeplete. Then you can download the latest version, and upgrade.
Hopefully this helps you out. I can say there is not huge benefit to purchasing vbgallery, if you already have photopost. Just upgrade what you already have.