Hi Stuart
It can happen because of various reasons... to name a few
1) Data type: When in canvas the datatype may be number and SUM on them work just fine. But in preview mode the incoming data in the same range may be numbers but actually text datatype in which case excel function will throw N/A error.
So check if the data is changed between design and run time, if so if the datatype remains same.
One way to check: within SUM multiply all the cells by 1
2) The range at which the Vlookup is looking at is not returning values. In fact, as suggested already use Index-match instead of Vlookup - you can use my post for reference http://scn.sap.com/docs/DOC-48706
Add one/more spreadsheet table component and map all the data involved and take a look at runtime.
Take an excel snapshot while in Preview mode (File > Snapshot) and post if you can. This will help others help you debug.
Thanks
Runali