As long as you have purchased your ringtones from Verizon Store you will not need a third party app to install.
Once you have gotten confirmation message of purchase, tap and hold on message
tap Save Audio then go to Settings>Sound>Phone Ringtone you should see the ringtone you purchased.
Now to find out how to install my Ring Back Tones LOL!!
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.