The answer depends strongly on what you are attempting to do.
The Mono Project (Ref: http://www.mono-project.com/Main_Page
]) allows you to run CLR-based applications on OS X. Mono's libraries, however, are not complete replicas of those on the official Microsoft .NET runtimes, and it's quite easy to make software that runs fine on Windows but fails miserably on Mono. The Mono Migration Analyzer, also known as MoMA, is a tool you can run against your .NET assemblies to learn whether they will run on Mono, and, in rough terms, how well.
One place where you're almost certainly going to get hit, given that you are writing a VB.NET application, is the GUI libraries. WinForms requires X windows on Mac, which is not installed by default. While Mono provides some facilities for making native GUIs using .NET, doing so would require rewriting your user interface more or less from scratch--and the main project to do that, Cocoa#, seems quite dead at the moment.
So, basically: if your application is not a GUI application, your chance of being able to run on Mac is probably fairly high. Otherwise, it'd probably be worth putting your efforts elsewhere.