As someone who manages multiple Multisite installations, I'm here to tell you the current per-site pricing structure for VaultPress is prohibitively expensive. The challenge for us Multisite users is that the market share of Mutisite installs is probably too small to justify alternate pricing.

While I'm sure VP is a fabulous service, paying an additional $15 per month for backups and such is hard to justify for smaller sites when shared hosting packages are typically less than 2/3 that price. The $25 per year that BackupPress is charging is definitely more appealing. I'll have to take a look at their service.

I'm with Donnacha, whichever company figures out how to provide excellent backup and security at a price that fits the hosting market (and has scales pricing to fit Multisite installs) will probably make a bundle.