Basically, “yeshivish” can either refer to a sociological concept or a religious one or a combination. It actually has many more meanings than Chareidi does, since you can choose the proportion and type of sociology and religion that you mix into your definition.
It is also something that one can have in various amounts. You can compare how yeshivish one person is to another (although you probably shouldn’t!) or talk about how yeshivish someone is (for example, “he is super Yeshivish” or “he is not so Yeshivish”).
The term “chareidi” is usually used in “black and white terms” (pun intended). Generally, someone is chareidi or he is not. That is unlikely to be the case with “yeshivishness” where it is more likely to go by degrees as opposed to “you are or you aren’t”.
I still haven’t gotten to the concept of “Chardal” yet….