That is a real vibraphone played by Bill Wyman, I'm guessing there's a slide guitar part that is tricking your ear in to 'hearing' it as a vibraphone note.
It's likely that Keith used his 1957/58 Gibson les paul custom in to a Vox Supreme for the main guitar part on Midnight Rambler. In spring 1969 he was also using amps such as HIWATT Custom 100, Fender Showman, Vox AC30 and a WEM Dominator so it could be either of those.