As far as installing software from .sit files goes, there are a few methods: there's an
hfs disk maker linked in these forums; just drag the folder over it, and it'll create an HFS (not HFS+) disk image you can drag over the Mini vMac window to mount it. Then you just run Stuffit Expander within the Mini vMac guest environment to decompress the sit file.
Alternatively, you can download
ImportFL which runs within Mini vMac, and lets you import any file from the host system while the Application is running inside Mini vMac.
We should probably get these two methods on the wiki if we haven't, as they are the two easiest

I've also been tempted to bug Paul to convert ImportFL and ExportFL into a fake Localtalk share extension a la UNIX folder in BII and SS.