I'll bet the new automation features in V5 depend on .NET.
Besides, if you've been running Vegas 4 without .NET, you've been missing out on one of V4's best features. You need .NET to run scripts, and there are hundreds of free scripts out there, written by fellow Vegas users, to do just about any task you can imagine. It's really as close to an "open-source" NLE as you're going to find.
Whether you upgrade to V5 or not, run do not walk and install the .NET framework. You won't regret it.