ISBLANK with VLOOKUP where lookup value has IFNA function

Unless something has changed in the newest versions of Excel, ISBLANK(VLOOKUP(...)) [or any function, for that matter] will return False. Spreadsheet functions must return something. Nothing in the spreadsheet repertoire (that I am aware of) will be recognized by the ISBLANK() function as blank. Empty string "", or space " ", or 0 are all non-blank numbers/strings. So, the condition part of your IF() function will always return FALSE, and it will always execute the value_if_false argument (which appears to repeat the same lookup).

What characters/values/conditions are you needing to test for? I could see something like =IF(OR(VLOOKUP(...)=0,VLOOKUP(...)=" ",VLOOKUP(...)=""),VLOOKUP(...,3,0),VLOOKUP(...,41,0)) working, where you test the result of the lookup function against the three possible "error" conditions and, if any one of them is TRUE, pull from column 3*. The main problem I see is that, where you are using full column references, you expect that the lookup range could be very large, and exact match linear searches (4th argument of VLOOKUP() is 0/FALSE) are very slow. And, where we may need to perform the exact same lookup three times to test the three conditions, this has the potential to become very slow. A lot depends on how big your real data set will be and how many copies of this function you will have, just know that it has the potential to be a resource hog.

Is it necessary to perform this test in this cell/function? It sounds like this test ultimately goes back to the IFNA() test in column AT of the lookup table. Could you take care of this in the lookup table? I expect that the current formula in the lookup table is something like =IFNA(lookup function,0). Could you change it to IFNA(lookup function,H2) [H is the third column of the lookup table]. By pulling the column 3 value into column 41 of the table upon error detection, the formula in the OP's cell no longer needs to test for the "error" condition. It can just pull the value from the 41st column =IFERROR(VLOOKUP($B$8,DAL_Data!$F:$AT,41,FALSE),"") without the need for the IF() test(s). Would something like that be allowed?

* -- I could also see formulating this formula as =VLOOKUP$B$8,DAL_Data!$F:$AT,IF(OR(VLOOKUP(...)=0,VLOOKUP(...)=" ",VLOOKUP(...)=""),3,41),0) where you move the decision part to the column_number argument.

As MrShorty pointed out, a cell containing anything - be it data or a formula - is not blank. The ONLY time a cell will be considered blank is if it has absolutely nothing in it.

Thats 1 of the main reasons I seldom use ISBLANK, instead I prefer to test for ="" This will work for both (truly) empty cells and formulas that return a nul string ""

can you provide some context (data) on how you are using this?

Is this what you need?

=IF(VLOOKUP($B$8,DAL_Data!$F:$AT,41,FALSE))="",VLOOKUP($B$8,DAL_Data!$F:$AT,3,0),VLOOKUP($B$8,DAL_Data!$F:$AT,41,0))