@Thomas10310 It depends whether you spawn vanilla SP vehicles, online DLC in SP, or addons. I don't really know about replace because i rarely install those.
If you know how to script spawn you should be fine. If not look for a persistence mod or a trainer that supports persistence. I use Menyoo but not to spawn entities except for testing before scripting. I think there is a database or persistence option but not sure.
Anyway check out persistence mods. There is also a vehicle addon spawner mod which i think will do this, at least for addons.
You can also try to save vehicles with your game but my experience is that this is inconsistent. I had an pn Ferrari saved and it was there every time i loaded story mode and then i session it was gone for good.
well everything works for single player vehicles and add ons, but for online vehicles it's random, I'll try another trainer , thanks !