Odd SYD-SIN Y fares?

[Spongbob]

Received an email from QF about a “Beat the crowd sale” (BTCS), showing Sydney-Singapore return economy (Y) fares for $699. While this is much cheaper then the non-BTCS sale fare of $683 on QF81/QF5 plus $658 on QF82/QF6 $658 (for a total of $1341 return), it seems odd that the same flights on the same days are much cheaper if the booking is to HCMC (SGN) in Vietnam via SIN. Here the QF81/QF5 ($493 inclusive of a flight on 3K551/3K555) and QF82/QF6 ($460 inclusive of flight 3K558) return fare is $953.
Given the 3K SIN-SGN return fare is about $250, it seems the BTCS fare is offered even outside the sale period (as long as you go to HCMC).
Could the SYD-SGN-SYD flights be an option for a cheap QF fare to Singapore the F1 Grand Prix in September 2014 (i.e. don’t use the SIN-SGN-SIN portion)?

 

[sergeyvzn]

if you don't use your SIN-SGN sector, the rest of your itinerary will be automatically cancelled

 

[QPProletarian]

What are the stopover rules? Could you go SYD-SIN-SGN-SIN/SIN-SYD