Again, there are mixed opinions. Here are my 2 cents:
1). VETASSESS website clearly states that
"You have received a Skills Assessment from a different assessing authority and have been referred to us by your skills assessing authority and/or DIBP for Points Test Advice on the educational level comparability of your overseas qualification only"
Now ACS won't assess a mechanical engineering degree so it'll the Case officer's call. He/she may ask you to get an assessment (if he/she is unable to verify. There are other sources too E.g.:
So you could either get a VETASSESS PTA or leave it on CO's discretion it's your call. My case is same as yours and I have decided not to get assessed.