The Mellotron Group

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. 

http://i51.photobucket.com/albums/f395/sundaramusic/The%20Rolling%20Stones/spring%201969/Keith1969a.jpg
Keith Richards - Olympic Sound Studios - Spring 1969

Here's my 1967 Vox Supreme: 
 
http://i51.photobucket.com/albums/f395/sundaramusic/Amps%20and%20effects%20pedals/1967VoxSupreme.jpg