If I remember correctly, the VPX files are used for verification to ensure the installation is not corrupt.
My setup is nothing like the size you mention, mine is a little over 500MB (345MB size on disk) and I installed Avast free on this system around this time last year and it has had a number of program updates since then.
What avast program do you have installed Free/Pro/AIS/Premier, etc. ?
What is the version number, 17.7.2314 is the latest ?
You could also try a clean reinstall and use a Custom install only installing those components that you need/want.