You are trying to control an application through its GUI, but it is not designed for that.
Having said that, I estimate this will be a nightmare if you do not use tools (or an app :-) ) explicitely created for that purpose, a la test robots like HP QTP or automation tools like Autoit http://www.autoitscript.com/site/autoit.
For example, if your application occasionally flushes the keyboard buffer, you might need to wait for some GUI event before sending the keystroke. This kind of synchronization is not accomplishable using vbscript only.