I think it could have been the temperature. If it gets lower than 75 problems can occur. When I got my first betta I had him in a small bowl with no heater. He seemed normal at first but then he was really lethargic and hanging vertically. After getting him a bigger tank with a heater, he was the mightiest betta.
It was a thought, though I figured you already knew they need surface area to breathe.
It was likely leftover residue or temp, then. Its hard to say which, although I think its probably more likely that it was something leftover that poisoned her in some way. Generally, a drop in temperature doesn't kill them - it lowers their immune system and causes sickness which can kill them. Since she had no signs of sickness, I don't think it was the temp (though it was possible).
I had a female pass away from dropsy after a temp swing caused by a power outage. I know that the temp swing is what caused it because she was totally fine before the outage, but that temp swing alone didn't kill her. It was opportunistic disease that grabbed hold of her after the power outage. I also believe that she didn't have the strongest genetics because none of my other four bettas got sick. Some bettas just have very bad genetics. Often, pet store bettas have worse genetics than bettas from private breeders.
There could have been other things here we didn't catch, but it was 99% likely that it had something to do with the vase she was moved to. Just in case it was some type of disease, I would disinfect any equipment you used with her that you are planning on keeping and using for your other fish.
I'm sorry for your loss. Don't blame yourself. Even the most experienced keepers deal with loss and sometimes our best just isn't enough.
Perchance, are there air fresheners in the room you kept your girl in? I've heard of bettas dying from being exposed to too many air fresheners(from cans) but I don't like to take my chances with candles either.
We've come to the conclusion that it must have been the waterbead residue that poisoned her. I do use incense in my room [where the fish are] but all of the tanks are covered, and when I ever do use air freshener I actually spray it at the floor! Can't take any chances.
What happened to Our little girl [Anima] was a harsh lesson we won't ever forget. From now on I don't think we will be putting any plants with our betta's we don't know are 100% safe.
I actually had another betta die a couple months back who was my favorite baby boy. A green MHPK named Malik. He showed no symptoms either, but he was also genetically defective- AND he had a close call with dropsy earlier in his life, so I thought the two things combined led up to eventual organ failure I don't think I could have prevented.
He hadn't been showing signs of ill health however. We just found him dead in near pristine condition. He was nearing maybe over a year old?
I don't know what happened either, but please don't become paranoid... And you are a good person who care about bettas, you were planning to buy her a new tank, so it is not your fault. Unfortunately sometimes we don't understand what exactly happened, we only have some clues about what it could be, but we have to accept this the way it is and be good betta parents from now on too. It happened to me also...