Guide for piezo charging?
Some manufacturers have created solutions for what you're talking about in the form of an IC.  Linear Technology has the following energy harvesting platform that may work for your application:

http://www.linear.com/solutions/5153

It's based around the LTC3331 energy harvesting IC.  It has the option to also use a supercapacitor in addition to a LiPO battery.  This may or may not be helpful, but using a LiPO by itself may run into lifetime charge problems, as many LiPOs are only rated for 100-200 discharge cycles before they are at half their initial capacity.

The solution above uses the V25W piezo from Mide Technology Corporation, which is now an obsolete piezo model.  The datasheet for this piezo is here:

https://media.digikey.com/pdf/Data%20Sheets/Mide%20Technology%20PDFs/Volture.pdf

So it looks like you'll have to find a suitable alternate piezo manufacturer, but it sounds like you've already got something in mind.