Hi and BTW I see we have another Dana on this post. My handle is @danab I find its not a popular name so I see so few people named Dana.
Anyway, as for the Vegas nerve it does have a lot to do with heartbeat. Im a transplant recipient and during that process they cut that nerve away from the heart. Nerves don't grow back and so it definitely affects heartbeat.

Before my transplant I delt with what your dealing with and it always seemed to happen about mid night. Now for me I already was diagnosed with sleep apnea and was using a cpap now apap daily. But I never tied the two together but it does make sense. Mine only got bothersome towards the end as it seemed my medications were not controlling the rhythm problems any more.

The sleep apnea does sound like a solution as apnea is where you heart stops briefly and I can see you body reacting to restart it and that may tun into a fast heart beat.
While Im on the subject I started out with just the nasal pillows but I also snore so I needed the full mask as i would just breath thru my mouth and the nasal pillows were ineffective. I went with the soft touch almost like felt as the mask seal and its very confutable for me. The other thing I found is I had to play with pressure and ramp time to get it to feel confutable. At first the ramp time was too long and I felt like i could not breath. So now I have it come to full pressure quickly. Changing the mask as often as your insurance allows for me also helps as the mask leaks if I use it too long. Just some things Ive discovered.

Good luck on solving the problem and if I can help please let me know.