vacant----something that is not being used or lived in,or a position in employment that is not at present filled.(空缺）hollow----be empty and with nothing inside（中空）blank----be empty and with nothing written on（空白）

I think...the difference is that they're used to describe different things.i.e. vacant is usually followed by post, job, situations, etc.e.g. vacant seat, vacant apartment, the position is vacant, etc.hollow is usually referred to the physical conditions...or geographical aspect.Blank is most commonly used, similar to empty. That's the way I used to distinguish this three terms, hope it will also be useful for you, too.

vacant----something that is not being used or lived in,or a position in employment that is not at present filled.(空缺） hollow----be empty and with nothing inside（中空） blank----be empty and with nothing written on（空白）

ＢＬＡＮＫ是填空的那种...ＨＯＬＬＯＷ是空白ＶＡＣＡＮＣＹ是...空虚？猜的～～

Thanks.