SQ generally default all meals on a flight to selected special meal, so you could try choosing BTC first and then the special meal for refreshment.
One disadvantage of choosing BTC vego option is that you are stuck with the inflight menu for starters and they generally dont have any meatless options for starters.
I'd go AVML all they way for SIN-SYD.
1 hour transit should be fine, you might get 10 mins in the lounge.
SQ dont do pyjamas in J, you can request an amenity kit for SIN-SYD sector
