I would like to know this answer as well, I have seen this behaviour (what the OP refers to and the post above) creep up recently. My car is full stock with ecuflash and lc-1 for testing/tuning. After dialing in the AFRs to 10-11 over stock im starting to see more vacuum and it drop off while lightly accelerating.

 

this is normal, my car is the same way. Im not sure about the slow spool though

 

my car used to have 18-20 vacuum.

after the cams install the vacuum dropped to ~12 , always in idle...

now when in gear and lift the gas, the vaccum goes to -20 or more i don't remember exactly... That's completely normal...

 

id like to know this as well. i know that the throttle body leaks a little bit and thats probably why it creeps back down to -20 when your in neutral and just hold a rev at 3000rpm. if not then maybe you have a leak.. which means i do too =)