Umm Virgin "propping up" Tiger ? What exactly is your source ? Did you read and examine the Half Yearly results ?
Tiger actually had the highest load factors on flights in the Virgin Group and with one of the lowest cost bases in Australia it is the profitable part of the group.
It's actually Virgin that is pulling down Tiger, Virgin posted a loss and in response they made almost no changes to the Virgin network but instead culled Tiger which will help their books look better in the short term.
If Virgin think that getting rid of half of Tiger will mean Tiger passengers will fly Virgin they are incredibly misguided, they have literally just handed Jetstar a Golden Egg.