I'm using an egg cup for the chicks water right now, don't want to put too much in. Couple of drops?
I do not think you should put vinegar in their only source of water. Give them a second one, and put the vinegar in it, if you want to use vinegar.
Also it could be the temperature. They are in my room and I keep changing it because they seem too warm. They don't even seem to need the heat and they're only just over 1 week old. [...] Their box has a tunnel going to another box.
Are you using a heat lamp, a heat plate, or changing the temperature of the entire room?
It would be best if ONE of their boxes has a heat source (heat lamp, light bulb, heat plate, heating pad). The OTHER box should NOT have extra heat, and should be a temperature that's comfortable for people (70 degrees fahrenheit is good, 80 degrees is OK, but try not to have it hotter than that. Cooler than 70 degrees is also fine.)
Chicks do best if they can choose their own temperature.
If you're trying to solve the pasty butt issue, I suggest you try giving them absolutely NOTHING except that chick feed and plain water, for several days. I suggest no oats, no greens, no yoghurt, no corn, no probiotics, no vinegar, no bugs, no treats. Nothing. (Yes, if they have grit, leave that. Yes, leave the dirt in the box, but do not add any more corn to it.)
Changes can upset a chick's digestion. So if you set it up for them to control their own temperature, and you stabilize the diet, and then don't change anything for a few days--I expect things will settle down pretty soon.
It's not bothering either of them in any way. It's more me worrying it could get worse.
Or you could just decide they are fine, and keep doing what you have been doing.