By setting the link to LAN in the NX Client GUI Configure -> Desktop tab, you will almost completely disable the NX X protocol compression.
To be sure that also images are sent in their raw format, you can set Use plain X bitmaps in the client GUI -> Configure -> Display -> Use custom settings tab.
To disable encryption, verify that the Configure -> Advanced -> Network -> Disable encryption of all traffic box is checked. This will require that your client is able to establish a direct connection to the server using the TCP ports between 5000 and 6000. You should configure your VPN software to listen at these ports and forward them to the corresponding ports on the NX server.
When disabling the NX X protocol compression you can expect the performance of NX to be greatly reduced. This holds true even if your VPN software is compressing the traffic by a generic algorithm. Some of the NX benefits will be still be made available to applications, as for example the reduction of X protocol round-trips, but the session will not leverage the advanced NX functionalities, like caching of X protocol requests.
While NX can reach a compression ratio of 50:1 and more, you can expect the generic ZLIB compression algorithm employed by most VPN softwares to exceed a factor of 5:1. The suggested alternative is that you leave NX compression enabled, by choosing the link setting that best fit the network characteristics, and only disable the NX stream compression.
To do that, be sure that the Configure -> Advanced -> Network -> Disable ZLIB stream compression checkbox is selected. In this way you will let NX use its advanced techniques and will only defer the final compression stage to the VPN software.