The mode a quickbook file is opened in depends on who opens it and how.
The most likely scenerio is
You log in under multi user mode.
You log out or get logged out because of inactivity
your partner logs in under single user mode
You need to have a talk with your partner and make sure that he opens the file in multi user mode. He may not notice because most of the time you have logged in first and as the second person logging in he is automatically in multi user mode. But if you have logged off or been logged off when he logs on it is up to him to put it in multi user mode.
That is not the way quickbooks works. The first person to open the file determines what mode it is opened in. If you were the one opening the file in single user mode it would be your partner that could not get in while it was open.
This page may be of value in showing how quickbooks is set up to host multi user mode files.
As you can see there is a way to set a particular computer or more than one to automatically open in multi user mode. The simplist explanation for the symtoms you have is that your partners computer is not set up that way.
Do you know what type of vpn you are using? There may be some slight chance that because of the way the vpn is set up it gives your computer a new id every time you log in and quickbooks no longer reckognizes it as a multi user host. But that seems very doubtful to me.
I'm a strong believer in cynicism ( although that doesn't always work either :p )
In this case from what you have told me i believe the problem is on your partners computer. If he can get in and you cant it pretty much has to be on his end. Quickbooks decides whether a file is single or multi user based on who opens it first.
I suppose the thing to do is wait for him to get back to his office.
I do hope that you will check back with me if it does work as i only get paid if you accept the answer. I will continue to work with you if it does not work either way.