First, apologies for the really delayed response - I'm now working 6, and occasional 7 days a week, that plus a 3 year-old bundle of joy can really hit the response time.
I can't recall exactly what MJD wrote on it, but Vampire Fleets, in the Migration sub-section on Page 8 referrs to pressure from the Puppeteers from coreward, who developed "somewhere near the old Imperial core in the 1140s.
Also, the entry on Khalid Kentaro in Star Vikings mentions the Unity of Promise collapsing under the first wave of Vampire Fleets in 1156 (fleets fleeing the Puppeteers?)