On MI you're *A status will do nothing nor will it appear on your boarding pass.
As a Velocity Platinum, your boarding pass will display "PTNR Partner Priority" on the SQ and MI boarding passes (which often confuses agents). If you were VA Gold or *A Gold then you'd get "Partner Gold" which seems to create a lot less confusion.
*A status also makes the computer display your "Lounge Invitation" printed on the boarding pass for the SQ flights. They will never make a call/have signs for Virgin Australia Gold or Platinum so listen/look out for Krisflyer Gold as they'll accept VA Gold/Platinum at this time.
Generally, they're quite good at recognising status and my experience has been...
Checkin - On MI use the Business Class line and on SQ use the Star Gold lane (which may also be the Business class line). No questions asked across Australia, Asia, and Europe.
Baggage - The extra 20kg automatically appears in the system no questions asked.
Security - On SQ, from Australia, they seem to give an express pass but oddly I don't get one when leaving Singapore (they hand out express passes at the gate whilst boarding to those "eligible"). Other places I've been given nothing but I'm not sure if anything extra was even offered.
Baggage - This has mainly been automatic but did cause some confusion in Zurich flying Premium Economy as she wasn't sure if to attach a Business Class tag or a PE one.
Boarding - In Australia (and some other ports in Asia/Europe) if they seem to make a boarding call for Business/PPS/Krisflyer Gold and *A Gold all at once and then use the Priority Lane. In Singapore, it depends but it's either a free for all if they use a gate where the boarding pass is scanned when entering the gate area otherwise it's quite well enforced that *A Gold, SQ Gold, VA Gold/Plat board with Group 3 after Business/PPS has boarded.
Lounge Access - In Singapore and Australia they seem to have a good idea (and probably meet VA Gold/Platinum cardholders daily) but other places seem to question it a bit more but it always turns green after they can the card. As I said earlier all eligable lounge guests (*A Gold/Business etc) get a line like "You are invited to the silver kris lounge" printed on the boarding pass but VA Gold/Platinum don't get this hence there is sometimes a bit of confusion.