Not to say this theory is stupid(because it very much is) you got to think this from Vettels point of view, Vettel has had alot of reliabilty issues yes, but he has also been outperformed when there has been no issues as well.
Also if a driver was made to perform less then a team mate im prrtty sure they would be getting public about it as it would be clear they wouldn't want to stick around long.
There might be some truth if Vettel leaves next year and leaves in a hostile manner but i honestly can't seeing this happen.
I think its just unlucky reliabilty combined with being outperformed.