This is actually more of a router issue, and I really have no clue where it should go. I've setup a PPTP VPN successfully, and when tethering through my phone, I can connect and disconnect from it as as many times as I'd like.
However, when connecting through my Linksys router, after each time that I connect, I need to reboot my server/VPN server. It's strange, and makes me think it's an issue with my router since that's not necessary when tethering.
Originally, I had to open up a port on my router to allow the connection to happen, but having experimented a bit more, than port being open on the router doesn't seem to make a difference. The first connection always seems to go through, whereas subsequent ones fail.
I can post config files and/or commands I used to install the server if you think it'd help.