VLOOKUP is not case sensitive and when searching for 15 character Salesforce Ids, you must force Excel to match on exact case. The simplest way to return an exact case-sensitive match is by using an IF(EXACT... formula, but this will return #N/A if the first ID found is not an exact match.
The best way to enforce a case sensitive search in Excel is to use the INDEX / MATCH combo as follows.
