I've also your first bug in avast 8.xx. I believe it was after I defragmented my HDD. I simply uninstalled avast using the avast uninstall utility and reinstalled from scratch.
When installing a new version, it's always best to uninstall the previous one using the uninstall utility and install the new one from scratch. It's a pain to configure all over again, but it's worth it.
The second error never happened to me, can't help you there.
As for virus Vault, open avast and on the left side you'll see statistics. Click it. The you'll see Component Status, again on the left side, click it. You'll have an icon named virus vault on the right. If you click it, it'll open the virus vault.
Sorry about my english. Also, my avast is in Portuguese, so I'm making a rough translation, but i hope it helps.