Zentyal VPN module, like for almost all other Zentyal modules and although relying on existing open source components, will not expose, through Zentyal GUI, all parameters and features this module can support.
I'll let Zentyal staff comment on the rational behind this (to me, balance has to be found between webmin like approach exposing everything but potentially complex and not able to bring consistency by itself and interface that not expose any parameter but making choices in the behalf of administrator.
If you want to deal with parameter that GUI doesn't offer, the answer is
here.