SUBSTR() function in IF statement
You can use CONCAT function to add this two strings on this way:




create table video(link varchar(200), title varchar(200));






insert into video values ('y_LjhCEhWiKXk', 'Bruno Mars - Just The Way You Are [OFFICIAL VIDEO]');






select title,
       if(substr(link, 1, 2)='y_', 
          concat('https://www.youtube.com/watch?v=', substr(link,3)), 
          concat('https://vimeo.com/', substr(link,3))) as link
from   video;



title                                              | link                                       
:------------------------------------------------- | :------------------------------------------
Bruno Mars - Just The Way You Are [OFFICIAL VIDEO] | https://www.youtube.com/watch?v=LjhCEhWiKXk



dbfiddle here