Display column header value if data is being pulled from specific column

I have two sheets in my workbook – Sheet1 and Sheet2.

Sheet1 has two columns and six rows.

    A        B
1 [MARK]    [LUCY]
2 100       120
3 200       220
4 300       320
5 400       420
6 500       520

[Sheet1]

Sheet2 has two columns that I'm attempting to pull data into from Sheet1 using an IF statement and VLOOKUP.

In cell Sheet2-A2 I am pulling in a reference from any cell in Sheet1 from row 2 down. So the following example will pull in the value of 100:

    A                 B
1 [VALUE]           [NAME]
2 =Sheet1!A2
3 
4 
5 

[Sheet2]

In Sheet2-B2 i'm trying to include an IF statement that says:

IF value in cell Sheet2-A2 is pulling from Sheet1-ColumnA, then print the value MARK in cell Sheet2-B2.

IF value in cell Sheet2-A3 is pulling from Sheet1-ColumnB, then print the value LUCY in cell Sheet2-B3.

IF value in cell Sheet2-A4 is pulling from Sheet1-ColumnA, then print the value MARK in cell Sheet2-B4.



And so on, so the result looks something like this:

    A           B
1 [VALUE]     [NAME]
2 100          MARK
3 220          LUCY
4 400          MARK
5 520          LUCY

[Sheet2]

I'm not sure if VLOOKUP is the right way to do this so any help would be appreciated. Thanks!