Making fresh custard with eggs and milk isn't really difficult, but takes time away from preparing other bits of the meal, and can go wrong (e.g. bits of scrambled eggs from too fast heating). Thinking about this question - Why do I need to use oven to make creme brulee? - about why crème brûlée is cooked in the oven rather than the hob, my question is: Is it feasible to make a pourable custard just by mixing eggs, milk (or cream?), vanilla and sugar and baking it in the oven? If so what time and temperature should be used? 
Since you're putting the cabbage in a vinegar/water mixture, you are not fermenting it as sauerkraut (at least not initially), which would keep for months.  Instead, you're relying on the acidity of the vinegar for preservation.  The 50/50 mix you have there is plenty acid (est. pH of 2.8 or so), so even if fermentation doesn't start on its own it should be safe.  But for how long? This turns out to be hard to answer, even if we narrow it down to the most common recipe similar to yours, which is quick-pickled spiced red cabbage.  Online recipes advise from 1 week to 6 weeks, and there's no reason to believe those estimates are anything other than a rule of thumb.  The USDA is surprisingly ignorant of pickling anything other than cucumbers. The leading preservation books are no help either: Putting Food By does not cover refrigerator pickles for some reason, and The Home Preservation Bible says "3 days" (which is a bit silly since unsalted raw cabbage will keep longer than that in the fridge). However, there is one major difference between what you're doing and red cabbage recipes: you're using a cold brine.  Given that, there's no reason why fermentation of the cabbage wouldn't start on its own; basically, you're making an extremely wet sauerkraut. This also means that seeing bubbles is not a reason to think your cabbage and onions has gone bad; that could be fermentation starting.  You'd need to check, and see if it still smells sour instead of funky, yeasty or moldy, which would all be bad signs. 