Rope or Soft Vangs come in a variety of sizes depending on the length of the boat. They are all pretty much generic in that you have two blocks; one is attached to the boom and the other is attached to the base of the mast usually in a 4 to 1 arrangement. The ideal angle for this is 45 degrees. Note: to get 45 degrees, measure from the bottom of the boom at the mast to the base of the mast. Measure that same distance from the mast along the bottom of the boom. That point to the base of the mast should be 45 degrees.
Most rope vang setups have a cam cleat attached to one of the blocks. The tail of the line would end up passing through the cam cleat and this is where you would make the adjustment for the vang and secure the line.
The blocks are best attached to the bottom of the boom and the base of the mast by a Snap Shackle. This keeps the blocks securely in place but also allows you to disconnect the vang at the base of the mast and reattach it to the toe rail when going down wind. This acts as a preventer to keep the main from being accidentally backwinded, causing you make an accidental jibe (which can cause serious damage).
Rope vangs are a great mainsail trimming aid. Used in conjunction with the mainsheet system and the main halyard or cunningham, you should be able to shape the main to the wind that is available so that you can get the best performance on all points of sail.
If you like all control lines brought back to the cockpit, you can accomplish that by using just two standard blocks, usually fiddle style block, bringing the tail of the line off the block at the mast, across the deck to a deck organizer or single cheek block, and then back to the cockpit where a number of different items or pieces of hardware can be used to secure it.