vim search and replace limited to the highlight in visual block mode

Question

I often have text in columns and need to replace some things without clobbering similar stuff on the same line... a simple example follows:

Answer 1

You need to add \%V to your pattern. From :help \%V:

Match inside the Visual area.  When Visual mode has already been
stopped match in the area that gv would reselect.
This is a /zero-width match.  To make sure the whole pattern is
inside the Visual area put it at the start and end of the pattern.

OP EDIT: the explicit solution is to to use : s/\%V8\%V0/81/g