- replace the double block at the gooseneck with a single block and only run the mainsheet through it.
- install a doubler bracket at the base of the vang (somethign like this: https://www.westmarine.com/seoladair-vang-purchase-doubler-with-shackles-16945669.html)
- use a block or low friction ring on the boom and run a new piece of line from the doubler, through the new block, and attach to the top of the 4:1
- run the tail from the 4:1 through the block at the base of the mast, the rest of the run will be the same (this is the same setup I have which will result in a 10:1.
As far as function goes, The biggest advantage I see out of a boom vang is the ability to eliminate one variable form mainsheet sail trim. Without a vang, the mainsheet controls leech tension and angle of attach (in combination with the traveler). If you can set the leech tension with the vang, the sheet and traveler can be dedicated to angle of attach. I'd be interested to hear how other people use the vang, but that may be best moved to a new post.