For me ‘signifie’ and ‘veut dire’ are synonyms. So there is no différence.

Il n’y en a aucune en ce qui concerne le sens lorsque l’on demande la signification d’un mot, d’une expression ou d’une unité de langage plus grande, autrement le verbe « signifier » est plus formel.

The meaning is identical but there is a difference in register.

Signifier is more formal than vouloir dire so the former would be more common in written material while the latter would be more common in spoken French.

Note that vouloir dire is an idiom in your examples but does also exist as a regular sequence where vouloir keep its "willingness" meaning :

Que veut dire Dimitris ? -> Il veut dire qu’il a compris.

Que veut dire/signifie Dimitris ? -> En grec, Dimitris veut dire/signifie "qui appartient à Déméter".