I should have made my original post clearer. I was using an adapter (stereo) for headphones that had a 3.5mm plug to go into the 1/4" socket on the Vaquero. I had 5 different adapters and all had the same problem. I spoke to Rusty, and he asked me if I had tried other adapters, and when I said they all did the same thing, he said send the unit in. But he also suggested I just cut off the 3.5 mm plug and put a 1/4" plug on. I didn't do that but tried another pair of headphones with a standard 1/4" stereo plug. Suddenly there was no problem. Apparently all the adapters were not great fits for the Vaquero socked jack. Thanks to Rusty, the problem was solved. re: o-ring. I will try to add an o-ring to the adapter plug to see if that helps.
Andy from Hillsborough