Update column based on input variable in stored procedure
I think your stored procedure looks great!
The only line that had me scratching my head is this one:
Execute ('UPDATE [TblActionsOldest] SET ' + @vFieldName + ' = ''' + @vValue + ''' WHERE RefID = ''' + @AlphaNumbericalRef+ '''')

I'm guessing those apostrophes are escape characters for asp.net but if that's not the case I would discard them.
Nice work!