Vegas will treat a .psd as a flat image, no layers, though it will handle the alpha channel (transparency) correctly. There's no direct interface, at least not in the sense I would use the term. Vegas is simply able to read a .psd file.

A programmer, Satish, is nearing completion on something that will allow Photoshop plugins to be used in Vegas, so we'll soon have another way to leverage Photoshop's power.