Hi guys,
there is an old bug where GRE TAP tunnel does not silent fragment packets when MTU is bigger than the underlay MTU. Bug is fixed in 5.10.10 and newer which fits with vyos 1.4.
Bug report: https://bugzilla.kernel.org/show_bug.cgi?id=14837
Please add following parameters to gretap / gre6tap interfaces to ensure fragmentation:
ignore-df nopmtudisc
Thanks!