I'm not sure what you mean by the vBulletin built-in photo uploading utility. I'm assuming you mean that one that allows you to upload a profile picture.
Both Photopost & vBadvanced Gallery are seperate album mods. I run a board that has Photopost installed, and there is no feature that allows link photos into a post directly. You would have to upload it then right click on it to get it's link location and use the [img] tag. It appears on the new version of vBGallery released they have the [img] tag with the link to the image right on the picture which means all you have to do it copy and paste it over. (Very handy)
I just came across this hack which does what i think your asking for http://www.vbulletin.org/forum/showthread.php?t=73040
There is no feature in Photopost that allows you to link there album to their profile. I've been looking around for it for a while now but no one at photopost bothers to make it, or explain how to make it. I ended up doing it myself by just creating a link in the profile, and also the drop down box of username in the postbit, to their gallery. This kind of ties in with Question 3.
Once again this would be wonderful feature, however I don't believe it exists in Photopost. I wanted to add a gallery button along side the other links in the postbit, ONLY if they had images in their gallery however since I installed photopost onto a different database, I didn't know how to pull the information from a different database and display the information.
Stick with vBGallery. Although the only hands-on I've had with it was from their live demo on this site, I believe vBGallery surpasses Photopost in intergration. Photopost is designed to run on about 4 different types of bulletin boards. vBGallery is designed to only run on vBulletin, which is why it is so nicely intergrated. Photopost intergration with your current style looks lousy, your board will never match your photo album. vBGallery's style intergration appears flawless.