Comparison with PHP Session Variable

From my experience trying to do similar things, this is what I understand the process to be.

1. PHP code executes and renders the page
2. F5 overwrites the session variables after PHP code has executed.

You can check out this question too for a little more information.

Unfortunately, the only way I've found is to handle anything like this through Javascript on the client side, though that doesn't give you the flexibility you're looking for (as you mentioned).

Another option you could consider use an empty event in the VPE and based on that session variable pick from different logon pages. It's not ideal since you'd need to customize both of them individually, but it might accomplish your goal.