If you have ever let a glass of milk sit out for a long time, it is the liquid that under the thick old nasty stuff.
No it's not vegan. One's opinion on whether or not it is worth avoiding in every
situation is a choice I do not feel is an end all be all. Such as promising to make a cake for your kid and the only stores open don't sell any margarine without a tiny bit of whey. "Whey" the pros and cons I guess.
That said, I generally keep an eye on which things have it, which don't.