He didn't vacate, you're right, I was wrong. He intended to do so though:
Following his dominant unanimous decision victory over Brandon Rios, Manny Pacquiao has now been ranked by the World Boxing Council as the number one contender to Floyd Mayweather's WBC welterweight title. While some might think that the ranking is a positive sign as far as a potential mega-fight between Mayweather and Pacquiao is concerned, promoter Bob Arum seems somewhat pessimistic about the possibility.
"I don't know whether that means anything. I have no idea whether we can move forward with Mayweather," Arum recently told Ronnie Nathanielsz of Philboxing.com. "Let's see what he [Mayweather] does as far as vacating titles or whatever. So unless we know what he's going to do there is no sense getting excited."
WBC RANKS PACQUIAO AS #1 CONTENDER; NOW THE MANDATORY CHALLENGER TO MAYWEATHER'S 147 TITLE || FIGHTHYPE.COM
World Boxing News communicated with WBC stand-in chief Maurcio Sulaiman who had this to say on the matter:
"The new ratings have been released with Pacquiao at number one, but the welterweight division is now in a voluntary stage and the topic of the mandatory will not be addressed for a few months by the WBC board of governors."
In other words, the mandatory will not be enforced for Mayweather's upcoming fight on May 3. Not that any of us expected it to be, but at least the WBC is being upfront about it. However, the real question is what happens after they both fight in the spring, assuming each man wins. Will the WBC actually enforce it?
Also, just one thing to keep in mind. If Pacquiao fights and beats Timothy Bradley he would regain the WBO belt which would, in turn, eliminate him from the WBC rankings. So I guess if you have any sort of hope that this WBC ranking will help make the Mayweather-Pacquiao fight a reality, don't pray for him to face Bradley in April. (*Newzz edit* Pacquiao DID fight Bradley, which is why Floyd didn't have to vacate like he threatened)
WBC: Mayweather is in voluntary stage
“Everybody's talking about the title he’s (Pacquiao's) the mandatory. He can have the belt! A belt don't make me. A belt doesn’t define Floyd Mayweather.”
I think Canelo/GGG negotiations are similar to Floyd/Manny’s situation