Checking a recent 1.5 build (1.5-rolling-202405090225), I do not see the discrepancy mentioned in the description: the result for 1.5 is also (just) missing vif entries, and one can observe that the syntax error in vif 2 causes cascading syntax errors in the other vif sections.
The following script recreates the effect of the XorpConfigParser:

Migration requires the config file to be in the standard syntax, such as produced by the system itself (set/del; commit; save) --- in this case, a configuration that can be loaded and saved on VyOS 1.3.5 would not be missing braces as above. If one needs to hand edit a config file, at the very least one needs to load/save on the source system before migration. The only weird corner case, linked above, is when a change of mac address triggers a rewrite by the ealry-legacy parser (only used in <= 1.3.x, and only in one remaining place in 1.3.x: vyatta_interface_rescan) --- that does produce incorrect syntax and is a known problem as described in the comment in T5611; that case is also restored by saving the config on the source image before system update and the resulting migration on reboot.