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Here is my take on both issues.
1. You need to file I485 with the service center where i140 is pending. INS should transfer your case to appropriate service center if it deems fit. If you file I485 with VSC, they will either send it back to you or send it to TSC...
2. The statement of that clause about AC21 485 portability starts with the words " certification or approved I140 is valid if 485 has been pending more than 180 days", So you need a approved i140 in hand and 485 pending for 180 days or more.
So if at the time of I140 approval your 485 has been pending for more than 180 days then you can claim AC21 and cahnge employer to take up a similar job.
If at 180 days of RD of 485, your I140 is still pending, you need to wait till you receive I140 approval to change employer to take up a similar job...
I would be interested to see everybody's opinions (especially fellow senior members) on the second issue...