Combine VLOOKUP with OR Function with three options

Hello! I'm frustrated because I cannot figure out how to do the following:
I want my VLOOKUP to bring back a value if it exists; if it doesn't, look in the same table for a different combination; if that doesn't exist, look on another tab; if it doesn't exist there, bring back #N/A
My formula is:
=IFERROR(OR(VLOOKUP(LEFT(A2,12),'SLIN rev'!A:B,2,FALSE),VLOOKUP(A2,'SLIN rev'!A:B,2,FALSE)),VLOOKUP(LEFT(A2,12),'UB SLINs'!A:B,2,FALSE))

It's not working....here's the breakdown of what I'm ATTEMPTING to accomplish:
VLOOKUP(LEFT(A2,12),'SLIN rev'!A:B,2,FALSE) = Lookup against this cell through 12 characters
IF it doesn't exist
VLOOKUP(A2,'SLIN rev'!A:B,2,FALSE)) = Lookup against this cell through the entire character field
If it doesn't exist
VLOOKUP(LEFT(A2,12),'UB SLINs'!$A:B,2,FALSE) = Lookup against a different tab (UB SLINs) and bring that value back

Any help would be greatly appreciated

EDIT:
I've attached a spreadsheet example