find with vlookup and replace the value

hey there,

i want to make it very simple.

example;
column a has unique numbers. (55000 rows)
column b has values (some of them empty) - 55000 rows

column c has unique numbers (column a also have those values) 613 rows
column d should be replaced with matched cells. 613 rows

here is the thing;
example - search C1 within column A. lets say result on 441st row. replace "B441" with "D1" and then search C2 .......

sorry for very bad explanation. i hope you gonna get it. need a solution very urgent.

best regards from a newbie..
Emrah