There is no built in command to start visual block mode in vim, but you can define one yourself:
command! Vb normal! <C-v>
Here is a breakdown of how it works:
command! Vb - This creates a command called "Vb". The
command means that vim will not throw an error if the command is already defined.
normal! <C-v> - This command tells vim to take all characters after it and act like you had pressed them in normal mode. The
! makes it so that all user defined mappings are ignored. This means that if
<C-v> is mapped to something else, it will still work the way it does by default.
Here are some relevant help topics:
User defined commands must start with a capital letter.
Also, there may be a conflicting mapping which prevents you from using
<C-v> to enter visual block mode. To check for any conflicting mappings for
<C-v>, you can run
:verbose map <C-v>.