Hi,
A good suggestion, rather than have you battery connected to Vin, there is nothing stopping you using the 5V from the IR DC-DC converter into the Vcc or 5V pin of the ProMini.
The Vin needs 7.5V or more, not 5V to operate as it is into a linear regulator and it needs more than really 7.5V to operate.
Using the DC-DC converter will be more efficient.
Tom... 
