When running on Xen, with interfaces provided by the xen-netback driver, sg offload must be enabled before the MTU can be increased beyond 1500 — otherwise setting MTU on the underlying interface fails and VyOS produces its crash report.
Discussed in forum with reference to relevant line 367 of xen-netback/interface.c
If VyOS can determine that the ethernet device being configured is a xen-netback driver, then VyOS should refuse configuration of MTU>1500 unless sg is enabled.
Alternatively, a dirty fix would be to emit a warning that sg might needed if the user is configuring jumbo MTUs.