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.