It's easy to get lost in a formula if you don't really understand what the formula is actually doing:-
The VLOOKUP function essentially takes a value that you specify, whether it be a hardcoded number/letter/text or a value within a cell reference, and then goes and looks it up in a table. It will look for that value in the leftmost column of the table, and either find it or the closest match, and will then return the corresponding value on the same row, in whatever column of that table that you tell it to:-
Example - With the following table
D E 1 1 0.20 2 10 0.25 3 20 0.30 4 30 0.35 5 40 0.40
and with your value that you are looking up in say cell A1 (and let's assume it is 25 for example).
You can put a formula in pretty much any other cell, that says, take the value in cell A1 (25), go and look for it in the lefthand column of the table (D1:D5), and then when you have found it (or the lowest closest number to it), go to the second column (or whichever one you specify if there are more than 2) and give me the number that it is on the same row in that column.
So, with the formula being =VLOOKUP(A1,D1:E5,2) it will first take the value in A1 which is 25, then go look for it in the leftmost column (D) of your specified table (D1:E5), and it will try to find that number. Now it isn't there, so what it will do is look for the next lower closest number, which in this case will be 20, and the 2 in the formula says to go and get the value in the 2nd column (E) in your table, that is on the same row as the 20. That value in this case is 0.3
If you put the value 30 or 31 or 32 etc into A1 now, you will see the result of the formula change, because now it will either find those numbers or the lower closest number (and in each case there it is 30), and will subsequently return 0.35 as the corresponding value.
Just to show you how the 2 really works in that formula, if you added one more column to your table so that it looked like this:-
D E F 1 1 0.20 0.15 2 10 0.25 0.25 3 20 0.30 0.35 4 30 0.35 0.45 5 40 0.40 0.55
and you actually wanted the value from Col F, then you would simpl;y change the 2 in the formula to a 3 to signify the third column, eg:-
With the examples already given, 25 in A1 would return 0.35, and 30/31/32 would return 0.45
The one caveat to all of this (When getting the nearest number is OK) is that the data in your leftmost column must be sorted in ascending order.
There are times when you would only want it to give you a value if you had an exact match on the number, and in this instance you would simply add a 4th argument of 0 or FALSE to the formula, eg:-
In these cases you do not need to have the data in your leftmost column sorted.