What free VPN server to use that would work with iOS?

Why not set up L2TP if you don't want to harden PPTP further? There are a plethora of guides on the internet on setting up VPN using AWS free service micro tier instances and Ubuntu.

Here is one article from the AWS support forum with a discussion and links to how-to articles for setting up L2TP over IPSec on Ubuntu:

https://forums.aws.amazon.com/thread.jspa?messageID=286936

For years I've gone through this cycle of trying to set up a home VPN (running on OS X or Ubuntu) that'll support client connections from both my Mac and iPhone/iPad, and hours later I give up in defeat. Six months pass and I think, "it can't really be THAT difficult" ... and the cycle repeats.

Anyway, a couple weeks ago I finally got everything working. Some genius posted exactly what's necessary to get VPN set up for iOS and it also worked with my Mac:

http://ubuntuforums.org/showthread.php?t=1645473&highlight=openswan+iphone