We manage our VPN through the PPTP VPN Server in Webmin. For some reason some of our users can log onto the VPN and some can't. I have verified that the Samba settings are the same for the users that can access and those that can't as well as the PPTP VPN Server settings. Using the same computer with the same VPN settings, I can login as one user but not the other. I cannot figure out why that would be happening. I have reset passwords as well as made a new "dummy" account that also does not allow access to the VPN. Not sure where to go from here and any help would be greatly appreciated!

The best thing to do is to check the logs. There has to be something there that gives a hint about the problem.

Other than that, a few things comes to my mind:
1) You have verified the settings for the accounts, but are you sure they have been created in exactly the same way? Maybe you have missed something, how about creating new accounts, carefully noting every step and see what works & what doesn't.
2) Passwords: A few times I have had problems with some characters in keys/passwords. Try changing password for a "not-able-to-connect" user and use only alphabetical.
3) "...can login as one user but not the other..." You did logout the one user first, right? What I mean is, you can't easily (if at all?) have multiple pptp connections open at the same time.
4) You try from same computer in same place?
The reason for this question is that pptp is a somewhat troublesome protocol, with many firewalls (Cisco ASA 5505 for example) you have to specifically configure it to allow GRE pass-through.

And last but not least: you should consider moving away from pptp, it is not a good vpn solution. The protocol is compromised, meaning it is not considered secure.
There are several other vpn solutions out there, personally I use OpenVPN whenever I can choose.

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The problem actually ended up being in the way the VPN server was set up. It was not authenticating passwords based on Samba like we wanted it to but instead authenticating against a CHAPS file that was set up. Once I found that file I was able to add the user.