:syntax on :set syntax=verilog
The syntax files are the .vim files, you can use locate to look for it.
In the directory, you'll see the list of syntax that your vi is supporting.
To make the syntax turn on by default to the language that you preferred, you can edit this file :
Put these lines into the .vimrc file.
syntax on set syntax=verilog
Thanks to YZ shared this tips with me December last year. :)