1) Welcome to the Party!
2) As you are obviously new here, I have some suggestions to help us all help you:
When you post, please post your entire configuration including (but not limited to) your installation method and vicidial version with build.
This IS a requirement for posting along with reading the stickies (at the top of each forum) and the manager's manual (available on EFLO.net, both free and paid versions)
You should also post: Asterisk version, telephony hardware (model number is helpful here), cluster information if you have one, and whether any other software is installed in the box. If your installation method is "from scratch" you must post your operating system and should also post the .iso version from which you installed your original operating system. If your installation is "Hosted" list the site name of the host.
If this is a "Cloud" or "Virtual" server, please note the technology involved along with the version of that techology (ie: VMware Server Version 2.0.2). If it is not, merely stating the Motherboard model # and CPU would be helpful.
Similar to This:
Vicibox X.X from .iso | Vicidial X.X.X-XXX Build XXXXXX-XXXX | Asterisk X.X.X | Single Server | No Digium/Sangoma Hardware | No Extra Software After Installation | Intel DG35EC | Core2Quad Q6600
3) This is a SUPPORT question, moving to the Support Board!
4) If you are dialing ONE call with vicidial vs dialing ONE call manually, your asterisk server is maintaining ONE call. There should be no difference in call quality. This is not related to CPU or anything else. Asterisk neither knows nor cares that the single call is generated by Vicidial or any other method.
5) That being said, were you dialing multiple calls with Vicidial? Were you using a different "Dial" string in Vicidial vs when you were dialing manually?
6) Making the calls via USB modem vs Broadband vs any other method is relevant only insofar as it implies Data speed and reliability. So: What is the speed of the connection? You need a specific amount of bandwidth per call and if it is not available, it will sound crappy or die.
Happy Hunting!