Would a OS VIE-BKK to LX BKK-SIN connection of 1:10h (undercutting MCT by 5 min) also be all right? The problem is that I'm on two separate tickets and at origin they might refuse to check-in my luggage all the way to SIN as I'm undercutting MCT.

I really don't understand why 1:15h should be required to transfer at a brand new airport. At Eurpoean airports MCT is often in the 30-45min range!