Just as an update, since VirtueMart Affiliate 4.0, it supports multi-tier functionality (up to 5 levels/tier), as well as individual commission rates for affiliates.

We're also working on VM Affiliate 5.0, which will allow individual commission rates per product, as well as many new powerful features, having most of the power of iDevAffiliate, but also taking advantage of its integration with VirtueMart and Joomla!.

You can test VM Affiliate 4.0 here:
http://www.globacide.com/vm-affiliate/live-demo.html

Warm regards!