I was struggling using vlookup against a list that exceeded the LVT and my custom CAML was not working until Alex told me I need to use a <Where> parameter in my CAML, like this: <Where><Eq><FieldRef Name=’ITRID’ /><Value Type=’Text’>[currentItem:ITRID]</Value></Eq></Where>.
Its now working as expected. Posting this in case anyone else runs into the issue.
Strangely, using the “Build Query” section threw a LVT error, but building the same query using the “Write your own CAML-query” section did not.