Not directly related to VyOS. I cleaned up and uploaded a little spare time project, an encapsulation overhead calculator: http://baturin.org/tools/encapcalc/.
Tunnel MTU/MSS is often a confusing topic for beginners, so I tried to make a visual tool to calculate it and learn where it comes from.
Would be nice to add more protocols and (ideally) also display frame header structure, feel free to add and send a pull request.