Eating pickled onions is neither healthy nor unhealthy as the vinegar virtually wipes out any goodness in the onion. Although there is currently some research into the anti oxidant properties of vinegar. That said, I'm afraid that a pickled onion does not count towards one of your five a day. Sorry
I would say that, although the vitamins might be gone, at least you'll still get the fibre and they are pretty low in calories. I'd keep eating them! (and they're blooming tasty!)
while they may be no points on the weightwatchers too many in one day is not good for the acidity of the vinegar or the smell factor of the onions. but have never heard of any serious harm coming from eating pickled onions