Thanks for the fast reply.
A backslash on it’s own causes the error – it’s expecting a valid escape code because the backslash is there. Backslash followed by a non escape code character generates the error message and kills the vLookup.
A backslash as a valid escape code ( \n \r \\ etc) doesn’t error, and the whole string is then successfully used in the vlookup query.
For example; domain\rtester works as \r is a valid escape code.
I’ve attached some more screenshots to help illustrate. Firefox gives a slightly different error message to IE, specifically calling out a “bad escaped character”