I know that great dragons have to have translators while in their native forms to communicate. But if they've shifted into their metahuman forms do they still require them, or can they communiate vocally on their own?

They can communicate just fine in metahuman form (hell, even in draconic form).

While in their natural form, they can use dragonspeech to 'talk' with anyone they can perceive. This 'speech' cannot be recorded or otherwise detected by technological means. That is why they have their translators. So one can vocalize what the dragon is saying via dragonspeech.

When they shapeshift or otherwise change forms due to spells or other abilities, they take on the speech characteristics of that form.

Change to a (meta)human? They can talk.
Change to a cat? No talking.

Dragonspeech is always available though.

The hard part is getting to talk to them in the first place.