You don't need a license for PPTP VPNs.
Also, the VPN license is only for dynamic (SSL and NCP client) VPNs. If you are doing site to site, no additional license is required.
I don't believe that the SRX platform supports PPTP at all.
Agree with JonesyComau here. The SRX cannot terminate a pptp vpn only ipsec or ssl vpn.
You can pass through the SRX via policy and/or destination nat to a pptp server. And that will not require any licensing.