CSV file transposed to ILookup - c#-4.0

Question

A vendor is providing us a csv file nightly. We need to take that csv, pull out some of the columns, then import them into our in-house application. The csv, as we receive it, looks a bit like this:
StudentId, GradYear, 2014 Thing1, 2014 Thing2, 2015 Thing1, 2015 Thing2
and so on, adding columns seemingly to perpetuity.
When I first saw this csv, my first inclination was to transpose it to 2 columns: thus:
StudentId, 123456
GradYear, 2016
2014 Thing1, overdue
2015 Thing1, completed
My inclination is use csvHelper to read this file in, and transpose it by constructing an ILookup. So I would have a Lookup that would look like:
var theLookup = ILookup<string, KeyValuePair<string, string>>
The key being the StudentId, and the KeyValuePair being the column header : cell value.
I can almost work it out using a Dictionary, with a counter for the dictionary key, but I can't make the leap to Lookup with the StudentId as the key.
Here is how I'm constructing the Dictionary:
Dictionary<int, List<KeyValuePair<string, string>>> vendorDictionary = new Dictionary<int, List<KeyValuePair<string, string>>>();
using (var fileReader = File.OpenText(sourcePath))
using (var reader = new CsvHelper.CsvReader(fileReader))
{
while (reader.Read())
{
var dynamicVendor = reader.GetRecord<dynamic>() as IDictionary<string, object>;
foreach (var rec in dynamicVendor)
{
var recordDictionary = dynamicVendor.ToDictionary(k => k.Key, k => k.Value.ToString());
vendorDictionary.Add(counter, recordDictionary.ToList());
++counter;
}
}
I can view the records with the following code. Do I need to do a version of this foreach to make into a Lookup? Or, should I just use a Dictionary with some "if it's 'StudentId' then make it the key?
foreach (var rec in vendorDictionary)
{
Console.WriteLine("Vendor Dictionary record # " + rec.Key + " values are:");
foreach (var value in rec.Value)
{
Console.WriteLine("Key: '" + value.Key + "' Value: '" + value.Value + "'");
}
}
Mostly I want to know if I'm even on the right track here.
When I have whatever object of vendor data I end up with, I will need to loop through it, compare student with our student record and make decisions about updating records in our app.