Viper is called Zerynth today: https://wiki.python.org/moin/Zerynth
They're similar, but not the same.
Zerynth is a development suite. Notably, it makes use of a VM.
Viper is just one of the code emitters buried inside the MicroPython source code, like here [0]. Notably, it produces native code, not bytecode for a VM.
[0] https://github.com/micropython/micropython/blob/master/py/ob...
It's developed for MicroPython, which does give them room for breaking changes, but has trade-offs.
Arithmetic is much faster, but dictionary lookups take much longer compared to CPython.
Viper is a code-emitter from a large subset of Python, and even allows for inline assembly. But it's only for a few architectures at the moment, like ARM and x86.