Manifold vacuum doesn't open the secondaries. Air velocity in the front half of the carb creates the vacuum needed to open the secondaries on a Holley carb.
You can see the hole air passes over, and even use an air hose to check the operation of the secondaries. If you look in the right front bore, from the top, there is a small hole on the back rear corner of the venturi. It's small, so look close.
If you can pass enough air past that port for it to over come the spring pressure in the can, the secondaries will open. Dan