Yes, wish I'd read this earlier. Booked with VA (and paid more), and wish I hadn't. Plat members on VA are worse off in terms of seating, etc. than people with no membership of either airline who book directly with SQ.
When booking with VA, you can't allocate seats. You need to ring VA and they'll tell you the number and booking ref for SQ. You call them, and they say you can't preallocate any of the seats that you'd want to allocate. They say they are only done 48 hours ahead, and that they'll note your preference. When 48 hours before comes around, I found you can't do allocation via the website. Because I booked with VA, those options are greyed out. (They would be available for anyone else that booked on SQ). Tried to call SQ and gave up after over half an hour of recorded messages with no hint of how long it might be, or where you are in the queue. Eventually got through to them the next day, and all decent seats are already gone.
What annoys me is that I could have purchased QF flights for much, much less (2/3 of the price) and had all the normal options available. Purchased VA tickets through loyalty but clearly a big mistake.
It's quite pathetic really. The airlines need to get their act together and fix the relationship/agreement.