I've flown on all 3 airlines in Y a lot over the last 2 years. Agree with all the above.
On board - neither SQ/EY recognise your status on board (and it's a good idea to show your card to them at checkin). VA will but it's usually just to say hello and give you first choice from the menu. You'll get opupped if it's rammed down the back on VA, but don't count on a Plat shadow as they continue to be terrible at that. SQ have consistently been as good as economy gets, EY can be highly variable, VA I don't really enjoy long haul. If you get a 10 across EY 777 then it is a tight squeeze and can feel claustrophobic. IFE on most SQ planes is great (watch out for refurbished 777-300s), EY is also pretty good, VA is the same as the domestic A330s.
Lounge - EY lounges are awesome. SQ is good except in SIN where it's ok but no shower or toilet. Transit in SIN is easy, at AUH it can take a while to rescreen depending on arrival time.
Ground - all offer priority check in and boarding, so no difference here.
Booking - you get more points if on a VA code, but if it's a codeshare then you usually can't do seat selection and don't get better seats. If I'm on SQ now I forego the extra points and book direct as then I can choose a seat & check in online, otherwise you could end up in a middle.
What would I do? If I need SC I book a flexy with SQ - cheaper than VA flexy, can manage my own booking, good on board experience, and plenty of flight times to choose from. If I prefer points then I'd book on the VA codes. If cost is most important then I just book the cheapest and also look at oneWorld options. I don't bother upgrading, and find the longer transit through SYD more hassle than it's worth so I avoid the VA flight these days - would make sense for someone in SYD though.
Hope this helps.