The block size will determine max size, of course. However, it is not uncommon to see a smaller diameter, single braid line in the tackle with a cover sheathed over the handled, cleated portion (the cleat being near the cockpit rather than part of the vang itself). The smaller diameter line in the blocks will reduce friction considerably with no significant loss in strength. So you could take your 7/16 line and strip the cover to expose the 5/16 core which will run through the vang.. the cover remains on the portion of the line which is handled and cleated. It would work well with cunninghams also. Many rope manufacturers will UV coat their products core for exactly this reason.. MDX, Warpspeed etc. The opposite method is to start with a UV coated single braid, such as amsteel (dyneema) and sheath the handled part with a less expensive polyester cover.