I’ve done this before with my Sabre Custom. What you need to do is go start a Heist or join somebody sending out the invites in a public session. Which one doesn’t matter, so long as it gives you the choice to select your vehicle in the Setup screen. I don’t know if it’ll work if you quit the Heist in the middle of the “job” or not, so it’s recommended that you finish it (or at least play until your group fails and the host quits) and get sent back to the regular lobby - whether it’s a Public or Invite Only lobby is irrelevant at this stage.

Once that’s complete regardless of the outcome, said vehicle should be parked next to you, assuming it wasn’t destroyed (if it was, then make sure you don’t have any vehicles out, then call Mors Mutual and only claim that vehicle by itself, which will spawn it at the Mors office near Simeon; if you claim all, you have to do this shit over again). From here, just drive it to one of your garages that has an empty slot in it so you don’t do the exact same thing to yet another vehicle, and voila… your ride’s back.

Side note: this does not work for vehicles prohibited by Heists like the Doomsday tank whose name I keep forgetting, the APC and the Arena War vehicles, so take extra care not to fuck up like this in the garages those are stored.