Thank you for the feedback. Currently, the primary reason for the password length limit is due to the restrictions of PPTP and L2TP with utilizing max 14 character passwords. OpenVPN supports longer passwords, but this would cause issues when a user would want to use PPTP or L2TP (including connections from the Vypr iOS app). The idea has been discussed to try engineering a solution for the PPTP/L2TP protocols.
For more information, you can visit the Ideas page for the feedback here, and feel free to comment or vote on the Idea: