
That isn’t quite correct, because you can take the address of a VB6 procedure and pass it to win32 CreateThread. See the related chapter in Hardcore Visual Basic for example. And of course you can use win32 mutexes and such. Thanks to the win32 interoperability, there aren’t a lot of limits of what you can do in VB6.
VB was great like you said until you needed to do something difficult. I remember Bruce McKinny’s Hardcore Visual Basic book being my best friend for a little while. It thought me I could do the hard stuff in C, make a DLL and drop that into my VB project.
Bruce had a saying, VB makes 95% of the job easy and the other 5% impossible.
My first job started out with mostly Visual Basic 6. One of the modules written in VC++ by a bunch of guys who were looking to get that on their resume (they had since departed). It probably should have just been in VB as well and was always a nightmare to debug and enhance.
Once .net was prominent we mostly moved to C#. There didn't seem to be much of a reason to go with VB.Net, C# seemed more "legitimate" and without the hassles of Visual C++/MFC.
Decades later and I still find myself writing an Excel macro in VBA now and then but I've largely moved on.
As a side note, there was a really awesome book I read back when VB6 was still new. It's called Hardcore Visual Basic, and it is about how to do things that were said to be impossible in VB. Using the undocumented VarPtr functions, calling Windows API, etc. It is out of print (obviously), but available online at http://vb.mvps.org/hardweb/
That page mourns the late VB6. It is right: VB.Net is an entirely different language. I don't even know where you'd get a copy of old school VB these days (though VBA in Office is quite similar). Anyway, VB6 is dead, so you are probably interested in moving on to something modern.
The nice thing about Python is exactly that everyone keeps going on about how awesome it is :) What this means for you is that people have written some really good libraries. Many of them are in the standard distribution and are well-documented. This is what Python people mean by "batteries included".
As for what I'd use for this app... Not sure. I haven't written anything GUI-heavy in a long time. With Java I usually hand-code the GUI, but there might be good visual editors these days. For Python there is an IDE called Boa Constructor which lets you build GUIs. I have not tried it though.