We finally found the leak!!!
We got a He hit on a drain from STG #1 Bearing Packing Head/LP Head drain. (see my comment on May 4, 2017 at 9:29 pm). The #1 bearing housing/head is located in the exhaust tunnel. We disassembled the housing/packing head and found all the joints were not sealed properly and the contractor didn’t torque the bolts to the spec and not enough RTV applied to the surfaces. We found no damages to the steam seals. Note: Our combined cycle was commissioned about 18 month ago.
After we started the combined cycle, our vacuum went to normal. This is a very hard place to find the leak cause there is a great chance there will be no hit on the drain. We believe the air got into the #1 bearing housing (which suppose to be under atmospheric pressure) from the oil lines cause we tested all the other connections. This is something for A14 users to look into in case they have vacuum leak.
I greatly appropriated you all comments and feedback.