Vim: insert the same characters across multiple lines

Question

Sometimes I want to edit a certain visual block of text across multiple lines.

For example, I would take a text that looks like this:

name
comment
ph         


        

Answer 1

1. Move the cursor to the n in name.
2. Enter visual block mode (Ctrlv).
3. Press j three times (or 3j).
4. Press I (capital i).
5. Type in vendor_. Note: It will only update the screen in the first line - until Esc is pressed (6.), at which point all lines will be updated.
6. Press Esc.

An uppercase I must be used rather than a lowercase i, because the lowercase i is interpreted as the start of a text object, which is rather useful on its own, e.g. for selecting inside a tag block (it):