Try poking around in the reportname.FRX metadata file to find a reference to vVar. While you can't really solve the problem there, at least it gives you ideas on why this is happening.

>While trying to solve a different problem with a report (see: 'Recalc report every time......' for today's date) I had the following wierd thing happen.
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>I deleted a control whose value expression was just a report variable, vVar. I also deleted the definition of vVar from the list of report variables.
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>Subsequent attempts to run the report (preview it, actually) failed with the error that vVar was undefined.
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>I looked high and low in my other controls to see if it was referenced anywhere and it was not. I looked to see if it was referenced by any other report variable, and it was not.
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>Finally, I did a select all, delete and also deleted every report variable. For all intents and purposes, the report was blank.
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>Subsequent attempts to preview the report still produced the error.
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>Go figure!