The only way TT will do Bali, is from Perth. Then, at the bottom of the pile you're competing with Air Asia. And you can't.
Or from ADL and DRW.
I am sure VA would want to retain a Business Class option on DPS flights as it is fairly popular and apparently mostly nearly full. Replacing these flights with Tiger would be self-destruction on VA's behalf. Tiger would be a last choice for most people, the damage to their reputation has already been done.