Have you tried naming the cell range? Worth a try. You can then use the name to refer to the range in your formulae.
I wonder if the problem is to do with using VLOOKUP on your mobile device. The version of Excel (I assume it's a Windows Mobile device) may only support basic functions.
On your mobile device, can you use the colon ":" in a normal sum calculation? (without VLOOKUP)... as in...
Does that work?