welcome to GoY. Yes I have in a large estate garden a few years ago. Not sure if it is a variety in its own right or just a fluke for this year. is this the only flower it has or are all of them like this? it will be interesting to see what it does next year.
how long and wide are the leaves? there are several species of vinca.

There is a Vinca minor called Bowle's which is very similar, but possibly a bit darker blue.

Nothing to do with Vinca but my clem Sieboldii is doing something similar, Rosypatio. Last year it flowered as it should but this year it's a greenish white with double flowers and no purple middle. I think the plant world is very confused this summer. Unless some of them are infected with Covid!

Thanks so much everyone. I have managed to identify it as V.minor 'Azurea Flore Pleno'. :)