The problem with writing to a text file is fighting over that text file.
If this is going to do millions of concurrent transactions (for example) text files would be BAD. Slow, only one process can open the file at one time etc.
you don't need to go into the windows messaging api
I can't remember how to do it in VB.NET (quit using it long ago, damn compiler at 1.5 gigs is way too heavy for my little laptop and doesn't offer me anything new for my purposes).
Your best bet isto have a primary application
Create a class in that application that holds the properties (values) you want. In that primary app have a public method available that makes an instance of that object and has the value set. If you come in from the 2nd app and call that method it should be able to reference the same instance of the object and hence its properties.
Clear as mud?

Hence I'm not a teacher
vb.net.. its a kind of object oriented language.. make use of it

or if its a very small light app.. youcan write to a text file *shrug* lol