It's a matter of "use the best tools available to you for what you're building". Mixing C# and VB within a project isn't recommended (for obvious "it won't compile" reasons), but there's no point in continuing to write old code in VB if you feel your development team can operate faster and in a more maintainable fashion using C#.
We've been doing this at my office for the past few months (in a similar legacy code situation) and have yet to run into any major issues (beyond potential lost dev time due to context switching), and we've gained incredibly from working with a language that we all feel more comfortable with.
More information on task switching here. I really do feel like the benefits we see on a daily basis from our level of comfort with C# outweighs the cost of occasionally having to dip back into the legacy pool.