One account. Each account may have an "agreed upon" maximum number of calls at any given time. Most providers will allow up to 20 calls without special arrangements (but never assume).
Vicidial will expect each campaign to have a single account to call through.
Your operators actually should not be making any calls. When Vicidial is set up properly, your agents will log in and just sit there. Vicidial will dial numbers, and each time Vicidial encounters a live pickup will immediately hand the call to your operator. This allows Vicidial to call MANY numbers simultaneously and keep your operators much more busy than if they were dialing one number at a time per operator.
When choosing a provider, you need to find out the price per minute and the minimum price per call, and the increments for billing after that.
ie: 1.5 cents per call ... 60 second minimum per call.
Also be aware that some providers will have issues with Dialers. You must be up front about this or you could face sudden shut-down. Also some have limitations regarding the percentage of calls that are answered (ASR=Average Seizure Ratio) and the average call duration (ACD).
For listings of providers, you are actually looking for SIP or IAX providers that can call Mexican phone numbers for the least price available and are known to work with Asterisk. Lists of them are available on many Asterisk support/technical/forum sites.
There is also a listing for those providing service for Vicidial, but this list may be a bit outdated:
http://wiki.vicidial.org/index.php/VICI ... _Providers (I haven't looked recently, so i have no idea if it is up to date at all)