Hello Dark Knight,
A 6 inch vise would be a useful size for your B-port mill. Keep a close watch on E-bay for a deal on a US-made 6 inch vise. The Anglok types are great but rather spendy.
I used a 6 inch asian vise on my US Machinetool mill. It worked OK for me. Also I used a matching pair of tilt-type 3 inch vises on that mill to secure long work, very handy.
Note that shipping a heavy vise can be more than the vise.