I've read through all the comparison charts, but I just wanted to make sure by asking here as well: is there something vbgallery has that photopost Pro does NOT have, that I would be missing out on if I chose not to use vbgallery (other than the vbulletin template system?)
There is no way to import from some external site like those which you dont have access to the database. You can only do imports from where you have database access and for scripts where there are imports written for.
Since there's a way that 'attachments' from vBulletin could be displayed somewhere like in the slideshow that I use on my homepage (vBSlideshow mod @ vbcover.com takes attachments in threads assigned to a particular forum and displays them), then why couldn't the same images be posted in a gallery mod?
Again, I'm not someone that knows much about coding, but that has to be possible.
Well I am sure if someone was inclined to write such a script it could be done. There is no such script out there so thats what really matters in the long run. You can not import forum content into a gallery. Usually looking at gallery import products companies release they write importers to import competitive gallery products into their product to make people to switch like say albinator, coppermine and gallery2 etc.
I have tried using 'Flickr' to import, I found I could do it using a PHP proxy file feeding a flash based gallery. I haven't tried this with Photopost Pro though, I used a method to read the RSS feed from 'Flickr' - but I couldn't get this working within Photopost Pro - I would assume vBGallery would suffer the same issues. Basically, you need to be able to read the 'Flickr' RSS feed, then either display the contents or copy them to your own gallery.