Sorry, this one slipped under the radar.
1) Installation is done via an install.php script - you'll need a server that supports the basic requirements, but the script will walk you through an installation.
2) Yes, keywords are part of a photo description which link to other photos with shared keywords.
3) Photos can be displayed in multiple categories (while only having one actual image stored on the server); keywords are a way of point to other photos which may be similiar, but not necessarily the same.
4) Yes, integration is a read-only kind of thing with no changes by out side. Skinning is as easy as flipping a switch and setting a path.
5) You can integrate the gallery forums into your vB forums just as if they are one in the same and PhotoPost comes with the scripts you need. The instructions are in the FAQ section for easy integration.
is a good example of an integrated forum with galleries listed in the forums (although I'm using the compact version in a sidebar there).