VectorRaster is a stand alone application designed to input a raster image, convert to a halftone effect, then spit out a vector EPS or PDF. No direct need for any Adobe application.
Phantasm from Astute Graphics is an Adobe Illustrator Plug-in that pretty much does the same thing within Illustrator. Phantasm offers more on-the-fly WYSIWYG control over the conversion as well as the ability to create halftones using special or unusual shapes for the "dots". There's really a great deal more control over the conversion in Phantasm, not merely a couple options.
I have both. I personally prefer Phantasm... but I'm a long standing Illustrator user as well.