I don't think she's had a tummy tuck or body lipo. She just worked really hard on getting her body back. In the three weeks prior to VS show..she was working really hard two times a day for one hour with David Kirsch and his work out is very hard. Rent his video and you will see. Heidi always says that there's no secret to loosing weight, its just eating right and working out. And, you can see that her weight is just belly.
Besides, some people are just lucky, I had a cousin who two months after giving birth was back to her regular shape. And, this is w/o doing anything.