In this case your vlookup is doing the work of a join (incidentally your vlookup is broken and wouldn't actually work anyway), the iferror covers the case where the formula cannot find your lookup value in the lookup table.
I've given you an example process. I invented an extra row of data for the sample.
1. Join Related_Case to Case ID
2. J Output anchor is where there is a match - renamed "Client Services Case Owner" field from right input as "CS Owner - Related Case" field.
3. L Output achor has no matches.
4. Union both streams back together, if "CS Owner - Related Case" is null then set to "0"
Regards,
Ben
Appreciate your help. Thanks