I have partially acetylated a hydroxyl group at high concentration of acetic acid before. It is not as effective as the anhydride or acid chloride.

Article A green route for the acylation of resorcinol with acetic acid

It's hard to answer your questions without knowing the specifics about your reaction, but usually acetylations using acetic acid result in good yields when you use it in high excess, high temperatures (more then 100 °C) and using a system to pull out the water generated during the reaction (assuming you are acetylating a hydroxyl group). Another important factor is the catalyst you are using, usually highly Brønsted acidic catalysts work well. Once again, it's hard to say more without knowing the reaction you are working with.

Particularly for acetylation of amino group.