You said it passed a vac test, I'm assuming you did it with the boot on, so it is ether leaking were the carb meets the boot or when you bolt it all up it''s pulling to one side and causing a leak at the cylinder flange. I would do another vac test and move the boot around a little while your doing the test. I would also check that the boot is setting right in the plastic flange. If it's leaking at the cylinder and the boot, and you can't get it to seal, you could try a little sealant around cylinder flange.