I'd suggest you include the Boomkicker in your list of possible solid vangs. They do the job for less expense and more reliably (no moving parts). You can get the standard Boomkicker from most chandlers. The rods need to be cut down to fit the boat but its easy. Boomkicker also sells a version just for the J30 with the right length rods to start with. The nice thing about the Boomkicker is that it works with your existing vang. Here's the link to Boomkicker's site.
http://www.boomkicker.com/