Several factors to keep in mind
1) Whether automatic conversion is 100% possible depends on how the VB.Net code was developed. A "looser" code style that would be unclear in C# is permitted by VB.Net. Therefore, some manual assistance could be required.
2) If the VB.Net code runs without errors, you might wish to leave it that way. Projects written in C# and VB.Net can coexist in the same solution.