In going back to Vendor A to verify quantity, is this considered a "Clarification" or is this considered a "Discussion"?
You're in a very grey area. Generally, "clarifications" are exchanges of information to confirm previously submitted information or to identify/correct minor technical or administrative errors. That would ordinarily be the case here- however, if the Government allows a company to revise it's proposal based upon the exchange of information, that is always a "discussion" within the meaning of FAR 15.303.
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If you allow the company to change it's price to accommodate a quantity change, I'd reluctantly call it discussions. If the company keeps it's overall price the same- and just says the quantity specified was the typo- then that is a clarification. In no circumstances should the unit price be allowed to change.
The GAO generally looks to the contractor's actions, not what the Government calls it, to determine which category (clarifications/discussions) applies.