A little mityvac doesn't have near the flow created by an injection pump.
I was thinking since there is no screen in the tank
small pieces could go down the line and bridge up where there is a large to small transition in the line.
Something a mityvac could easily pull the small amount of fuel through.
Ironically the MityVac pulled more fuel faster through the clear filter than the IP did. Maybe that just indicates that the vane pump was going bad.
I've found several other issues in the meanwhile and have the engine mostly torn down while waiting on the Giles pump to get built.