Vlookup Question-how do I manage the Vlookup?

Hi, I am trying to do a Vlookup from Doc A to Doc B. I have that sorted out. However, if Doc A changes, how do I manage the Vlookup?

The number of columns and the placement of columns won't change. Neither will the document name. However, the number of rows within doc A may change and so may the value within each cell.

I presume that the changing value of the cells will be fine, but how do I manage the number of rows changing?

Thanks