A little more description of the problem might help.
Vlookup is sensitive to data type. In other words, if the import from the other program brought in fields that look like numbers, but are really text, Vlookup won't match to a number. I run into this quite a bit with the data imports from our business system. You need to convert the number you are matching to text. You sometimes visually see that a number is really text according to Excel by watching whether it is right or left aligned in the cell.
Similar issues can happen if you are trying to match dates to a field that looks like a date, but is really text.
Omar Freeman
Kitchener, ON