I use both. For a complex mix I'll use Nuendo. For simple mixes that were actually cut (picture-wise) in Vegas I might choose to stay there. Nuendo competes directly with ProTools (which I also use) in terms of functionality. Vegas doesn't. Like anything of that complexity, the learning curve for Nuendo is pretty high. Are you doing the mix yourself? If so, go with what you know. If not and someone else is mixing for you, go with Nuendo. More general flexibility.