As MichaelB indicates, if you're using Vanguard's automated service and don't want to take an RMD, you'll need to work with them to cancel it.
If you're not using the automated service and don't want to take your RMD, you can just not take it. There is no need to notify anyone or do anything.
You can also take less than your RMD amount if you wish. You can also, if you wish, do QCDs from your first dollar this year since RMDs are not required.
"At times the world can seem an unfriendly and sinister place, but believe us when we say there is much more good in it than bad. All you have to do is look hard enough, and what might seem to be a series of unfortunate events, may in fact be the first steps of a journey." Violet Baudelaire.