Assuming you’re talking about the same thing: IPD in the DCS VR setting is not the same thing as IPD in the headset (unless they changed it in 2.7).
It works more as a cockpit ‘scaling’ factor - makes the cockpit larger/bigger or smaller. Or I guess it makes you smaller/larger. I didn’t grok this for a long time either. There’s been several threads on this in the last couple of years. Assuming we’re talking about the same thing (and again they didn’t change it in 2.7).
I leave it off and in the acft I use most it all appears ‘normal’, to me; Hornet more ‘roomy’ than the Viper for instance. It’s been said some, probably older FC acft, are off here and you need to adjust. Thing is, I don’t know what to adjust them too, given my understanding of this setting.
I dug this back up: