Really? I don't find this definition of account:
On which website? Because un vez isn't Spanish...
Well, as far as I know, mine is not literal translation. But it seems (I checked again and this time saw it wih había) that both érase una vez and había una vez are correct.
A native Spanish speaker could explain the difference if there is one?
I expect there are various ways of getting an equivalent idea across, as with many idiom and stock phrases. Have fun with it.