Make cell equal to contents of another cell

[icissel1234]

This question is making me think I've gone crazy, seems simple but can not figure it out:

If in cell a2 of sheet 1 I have some formula that returns a number by searching one database and if the value isnt there it searches sheet 2 'missing prehospital...' so cell a2 in 'missing...' has a validation list where i choose a text string and a vlookup returns the associated value but this only has to occur if the value was not available in the database

Formula in sheet 1 a2

=IF(OR(ISERROR(INDEX(MATCHAGEUNIT,MATCH(A2,MATCHCC,FALSE))),INDEX(MATCHAGEUNIT,MATCH(A2,MATCHCC,FALSE))=""),VLOOKUP('Missing Pre Hospital Data'!D2,AGEUNITSCODE,2,FALSE),INDEX(MATCHAGEUNIT,MATCH(A2,MATCHCC,FALSE)))&""

I would like all of the cells in the 'missing...' sheet to populate with the text string that corresponds with whatever value is in the same cell in sheet 1, and then if it does not exist in the database it will remain empty in 'missing...' until i choose a selection from the drop down

obviously this creates a circular reference if i set a2 in 'missing' =VLOOKUP('SHEET 1'!$A2,AGEUNITSCODE,2,FALSE)

is there any way around this?