Marc, if you're interested I can send you two classes that might help. I don't know what your entire code is for implementing this MaskedTextBox, so what I have might not be similar to how you implemented yours (in which case, it probably won't help, unless you use mine instead).
I can't really post the code, it's quite long ... even if I strip out some of the stuff that's particular to my Framework and not necessary to the functionality. Basically there are two classes a BBTextBox sub-classed from TextBox and a BBMaskedTextBox, sub-classsed from BBTextBox. It's old code, and I haven't used it in awhile, but AFAIK it still works. Also, it's in C# so you'd have to convert to VB.
So, if you're interested I'll email it to you ... just let me know.
~~Bonnie