The ixgbe driver did not support the 1000BASE-BX standard so for example FS.com SFP-GE-BX 1310/1490nm 10km transceiver received an unsupported module error even with allow_unsupported_sfp enabled.
To solve this problem I created a patch that was accepted by Linux upstream (https://github.com/torvalds/linux/commit/1b43e0d20f2d007ec4c124b0deaa848ff8d61f4a) so starting from kernel 6.9 the ixgbe driver will have 1000BASE-BX support, however VyOS uses the out of tree driver so it is necessary to backport the patch.