Decidedly odd, is it always exactly 80%?
Normally an iPhone vibrating endlessly while charging could indicate a few things such as water damage, a bad battery, a faulty screen or a badly earthed/faulty power supply. Though that's by no means an exhaustive list.
Try the easy things first which are, hard reboot the device by holding the home and power buttons until the Apple logo appears then release the buttons.
Then use the iPhone until the battery completely drains, without any top-up charges, and switches itself off.
Then use a verified good quality power supply and charging cable (not some random cheap off-brand one from eBay or such) make sure it's either official Apple or MFI certified, which many sold on eBay claim to be but actually aren't at all. Anker are a good choice if you want reliable but good prices.
Beyond that, take it into an Apple Store for them to hook it up to their diagnostic system. They'll find any actual hardware issues that may exist and are usually very good at offering you a replacement or at least a decent price for a repair that you can rely on.