1) Ports needed. VoIP traffic requires 10000-20000(udp) and 5060(tcp). Unless you have absolute control over your VPS firewall (unlikely) you will not even get a dial tone.
Except IAX which only uses one port and can break through firewalls fairly well. Also, if you are paying for a dedicated server you should be able to open any port you like (for instance a dedicated server at a facility like Atlantic.net). In which case there is no firewall, you must supply your own, and this obviously gives full control over it.
2) Bandwidth. Not to be confused with the amount of traffic they give you per month, this means the amount of bandwidth you can utilize at any given time. For example, an single VoIP call uncompressed will require 87kb up and down for the entire call.
Absolutely, whether you have a co-located box, a rented box, or a local install BANDWIDTH and CPU are the two primary factors to consider. You must make sure you are realistic in your expectations based on your available bandwidth.
3) The hardware being recommended by posters are fair for physical installations. How ever, a VPS requires less actual hardware specs due to virtualization being much more efficient at utilizing the system resources.
I belief that RupertMann is expecting a "dedicated" box to be ... a physical machine dedicated to HIM. Not a virtual box. Are you saying that you are renting vicidial installs successfully inside virtual boxes? That sounds cool. How many agents/lines can you support with an installation?
All valid points worthy of discussing before an installation "goes ahead".