Vicidial has multiple reports capable of checking this information from different viewpoints. Note that it IS possible to call the same number three times and make three sales. Thus vicidial allows this to occur.
If your goal is to check for sales for agents, you may wish to look at a different report that bases its statistics on the Lead data (ie: each LEAD may have only one Status) rather than the Agent Log (since each CALL has a Status there, instead of each Lead).
More to the point (from my experience) is that you should never have allowed agents to manual dial in the first place. This situation would not be in play, and agents would be less able to "game the system". In a professional environment you have agents with integrity AND your environment should be designed to make it virtually impossible for the agents to "play" even if they don't have an honest bone in their body. This way the dishonest agents can make YOU money just as much as the honest ones and you no longer have to make that judgement call. Just sayin'
alternately, of course, you could make "SALE" a DNC disposition and enforce DNC on manual dial calls. They'd never be able to call a SALE after the fact (from that campaign).