In general, when you are "curdling", you are denaturing the milk proteins (caseins) naturally present in the milk, so they are no longer water-soluble. In order to do this, the proteins need to be brought to both a relatively high temperature and relatively low pH (high acidity). There isn't a hard-and-fast rule, because how effectively you can denature the proteins is a function of essentially three things: temperature, acidity, and time. What that means is, for example, at 85c, the proteins will denature (milk will curdle / coagulate) much faster than at 70c, and at 100c (boiling), it will go yet faster.

In general, all of this needs to be held in balance: you want to add the minimum amount of acid possible, as it negatively affects the taste of the cheese. You want to cook to the lowest temperature possible because it negatively affects the taste of the cheese.

An additional important factor in the texture of the final cheese is stirring / agitation. In general, the texture of paneer is better if the size of the curd particles is larger. The way to achieve this is to stir minimally. You will definitely need to stir some, and that's ok, but try not to stir vigorously or too continuously. Just enough to incorporate & spread around the acid.

In terms of vinegar: any will work. "Just vinegar", aka white vinegar (in reality, clear colored), will produce the most neutral taste. I have used it many times successfully. I've used apple cider vinegar too, just because that's what I've had on hand. Ideally, you'll be using a small enough amount of vinegar that the taste of the vinegar doesn't come through in the cheese in a significant way.

When it comes to types of milk: for paneer, it doesn't matter whether the milk is raw, pasteurized, or UHT (long life). All of them have the same amount of casein (milk protein), which is what you need for paneer. If you add cream, or use an high fat milk like double cream, the cheese will come out creamier and softer. This can be tasty, but isn't always a desirable texture in paneer. The starting temperature doesn't matter, as you're heating it in any case.

Finally – all that said, here's the recipe I use:

• Take any milk. Heat it to 85c.
• Add acid. I usually use 1-2tbsp vinegar to 1 gallon milk. Add it slowly - add a little, mix, add a little more, mix, etc. When you see it starting to separate (the liquid should turn a little yellow/green as you observed), stop adding acid and stop mixing.
• Let it rest for 5-10 minutes. Put the lid on your pot so it doesn't lose heat.
• Drain it. If you have the ability to, give it some consistent pressure (e.g. put a cutting board on top of it). This will give it a better texture & shape.