It will be important to know DDS own OIP3. We remember that N2PK VNA has abt. +44 dBm its own OIP3. DDS output level is variable from abt -6 to abt. +6 dBm. For testing purpose we will use known connection:
DDS - BRIDGE - ATT - SA - PC
We can not forget that Reflection bridge has 6 dB of attenuation. First we will check IM features for minimum output level, then for maximum output level. See the following pictures.