Are you overwriting your old config before an upgrade? Because the upgrade instructions are explicite not to do this.
On a new installation you input your cookie settings etc and
VB license number etc upon install.
I have done an upgrade and an install and in both cases everything is fine.
The only issues reported and solved is alot of users where overwriting there config files on upgrade which is a no-no.
Upon an upgrade you would do two things.
First go to admin options and input the proper cookie variables under cookie settings.
Second go to config options and save that to make the config save as a new one.
On an install all values are entered by what you place them to be when installing. If you set VB3 integration to yes then we bypass the photopost cookie settings and use your
Vb cookie settings so there the automatic login is full proof IMO.
If you are deviating from the normal upgrade instructions that is where you could have had issues or on a new install not entering the proper cookie stuff then I can see you having an issue with cookies.
Even more so if you are using the VB3 enhancements and we are using your vb3 cookie settings I fail to see how there could be an error at all.