The Vektor is a mic not a piezo. It requires power which can be provided with a battery or is available in a Phantom powered configuration using a small K&K (3" X 2" X 2") preamp. (See
http://talkbass.com/forum/showpost.php?p=6515177&postcount=11) I use it in the latter configuration, as I prefer to avoid batteries.
There is no need to blend this device with a Piezo unless you like the tonal characteristics of a piezo pick up. Although it is a microphone, the "old" Vektor is embedded in the foot of the bridge and does not make contact with the air but, rather, is pressed against the top table of the bass using rubber-like spacers. So, creating a feedback loop would involve having the body of the bass act as an acoustic antenna to pick up the signal from the amp and feed it back into the Vektor. My experience has been that it is not very prone to feedback.
In the version I have you can use any amplifier that provides a microphone input with phantom power. (I forget the range of voltages that the K&K will appropriately convert to the required voltage, but certainly 48V works, and some lower voltages may as well.)
Jim