"does this mean that software written using VS.NET is less secure than software written using VS6? "
I don't think you can really make the claim that a program with the source disclosed is as secure or less so than a program with the source undisclosed. At the very least, it is a diagnosis made on a program by program basis.
Also, we are talking about VB - not necessarily enough complexity in memory management or system process to introduce significant problems (other than bugs, and the thing not working) at levels destructive to the system. Furthermore, many of the components are intrinsic to Windows and their source is hidden.
Additionally, .NET has managed code, so many common faults, like buffer overflows, can be avoided at runtime.
Frankly, you have to remember several things.
1) The decompiled code is not verbatim the interpreted source. Someone has to know what they are doing - this would disclude the average user entirely. And of course, not everyone has the decompiler. So we are talking a small group of individuals who not only have to care, but have the time and knowledge as well. You might wish your code carried a big enough incentive for them to even look at it.
2) You have to get the bytecode to decompile it. Sure in things you develop for local use this can be an issue, but on the server-side nobody cares. There should be no remote access to memory or read access of important files anyway. A lot of .NET is for server processes.
3) For many things, it really does not matter. If you have a small utility or similar functional program, it is not worth anyone's time to decompile it. If someone wants to, let them hack a local program. They will not be getting anything special. And without significant revision, it probably will not work outside of your intentions.
Where is this bad? Networking enabled clients. If a killer app with .NET was poorly configured with networking, a virus or worm carrying a malicious patch could sweep the Internet and make pernicious changes (to create zombies or maybe propogate something else). Unlikely though.