There is a good monograph(?) by AIAA written by Albert C Piccirillo titled "German V/STOL Fighter Program: A Quest for Survivability in a Theater Nuclear Environment" (1997). This is 54 pages long and deals with both the VJ101 and Vak191B. Text it interesting and informative, photo quality sucks but drawing are clean. It only refers to the 191 as the B, never a Mk1-3. Additional reading list in back though includes another publication by Piccirillo titled "Case Studies in VTOL Development: Volume One, The German VAK 191B Transonic VTOL Strike Fighter", ANSER, 1995. Since this additional publication was printed two years before the first title noted, I would assume most of it was included in this publication but can not be sure.
To put some of the above entries into perspective, the studies began in 1964 and the Italians dropped out in 1968 for financial reasons. By 1973 all three aircraft were testing systems for the proposed MRCA, which as the Tornado would eliminate the need for the VAK 191. But in 1973 through most of 1974 an additional 60 flights were undertaken with the US Navy.
(From Aviation and Marine, 1975) Prior to the termination of the program a Mk 2 was proposed with a 30% more powerful cruise engines, and 5% more thrust in the lift engines, a larger wing of 7.5m of 19 sq.m.