Joining the results of a LOOKUP or VLOOKUP

How can one join/concat the values of a VLOOKUP?

I have tried =CONCAT(VLOOKUP()) and =TEXTJOIN(VLOOKUP), but they only produce a single result.

Your VLOOKUP() surely must be more detailed. Please don’t suppress useful information.
I suppose you needed array evaluation to get a result by VLOOKUP you can pass in a useful way to CONCAT or to TEXTJOIN.
{=VLOOKUP(A1:A5;B1:D20;3;0)} would retur an array, e.g, if some content of a cell in A1:A5 finds a match in B1:B20 …
Please note that the present implementation of CONCAT and of TEXTJOIN is buggy in a sense. Come back with further questions and supply more details.

In Calc, VLOOKUP only returns one value. Sounds like you want something similar that returns multiple values; I’m pretty sure there is no such built-in function.

If you’re into writing macros, it wouldn’t be difficult to write a custom function that does what you’re asking for here.

Just made this little demo.