It would be good to know the differences between VB, VBA, and VBScript in the functionality available and the constructs of the language. All you're going to see is a reduced set of functionality and capability as you slide down the scale. VB (VB6 & .NET) gives you the most capability, VBA is embedded inside the MS products and is close to VB6 (just can't independently compile the code and include some external resources), and VBScript is the stripped down free version of VB that runs on Windows (XP, 2000, 2003).
One good thing is that you can transport code snippets from one to the other with modifications, mainly in variable types and some logic constructs. As part of working with QTP you will learn VBScript, and some of the other things that can be done via Windows Scripting Host (WSH). There is a lot of code snippets out on the internet for VBA, VBScript and WSH scripts that you can use in your testing (for file handling, working with the Registry, ODBC/ADO calls for database, CDO for email via SMTP, XML data manipulation via DOM, IE calls via DOM, etc.). See the possibilities. Google is your friend in this case.
Well VB knowledge definitely helps, but in this case your VBScript should be strong to make more efficient use of QTP. I have been working on VB from last 6 yrs and it has really helped me in learning QTP and VBScript