de :help CTRL-V-alternative
:
*CTRL-V-alternative* *CTRL-Q*
Since CTRL-V is used to paste, you can't use it to start a blockwise Visual
selection. You can use CTRL-Q instead. You can also use CTRL-Q in Insert
mode and Command-line mode to get the old meaning of CTRL-V. But CTRL-Q
doesn't work for terminals when it's used for control flow.