@xode,
Why do you need to download the complete VPS? The built in definitions update is very small (between 10KB and 400KB).
And when, for whatever reason, you need to do a complete reinstall, you need to have an update file.
That is not necessary at all. When you download the setup.exe for Avast to reinstall, it already includes one database. If you download the complete VPS again today, and reinstall Avast next month, the VPS is also "old".
After the (re)installation of Avast, you should reboot immediately. After rebooting and connecting to Internet, Avast will try to update the VPS. If you follow your original intention, you are downloading the VPS 3 times by then.
So I would say that you have no "real" reason for downloading the complete VPS now.
Moreover, using dial up, you download several KBs of data in every definitions update, which is more adequate for your type of connection. If you happen to have some download problem with such little download, then downloading the complete VPS is not going to be easier (with that same connection).