If you are still need help in this area check out this article...Doing ASA Quality of Service (QOS) on DSL or Cable Internet

This may be just what you are looking for.  According to this article, you have to tell the ASA what your DSL speed is so it can properly do QOS.  This guy doesn't talk about doing it over a VPN but I don't know if that would be much harder to implem...

Here's a very simple way to just do outbound QOS for VOIP over the internet. http://brian-kayser.blogspot.com/2010/10/doing-asa-quality-of-service-qos.html