Question

I select a block of text using Ctrl–V, but when I then type Shift–A or Shift–I, my vim editor goes into insert mode on only the location where I began the visual block mode.

For example,

Here
is 
text

I want

#Her 
#is 
#text

but I get:

#Here
is 
text

Answer 1

Ctrl–V-> select the block -> press I -> type #

then press ESC

more detail:

:h v_b_I

you can see:

Visual-block Insert                     *v_b_I* 

With a blockwise selection, I{string}<ESC> will insert {string} at the start 
of block on every line of the block, provided that the line extends into the block.