Since Sam and dean are both angel-capable vessels, could they slice an malaikat and steal they're grace and get all amped up?

I don't think so, I think only an (ex-)angel could take on another's grace. They can only be inhabited oleh an actual angel, since they're vessels.

As far as I know, only an actual malaikat atau former malaikat can steal and use another angel's grace. A person who's simply a potential vessel can't use grace, so Sam and Dean couldn't.