The initial version of freeradius-client, which kamailio was cross-compiled against, had the vendor id shifted by 16 bits. It was changed to 8 bits after https://github.com/FreeRADIUS/freeradius-client/issues/72 which as stated was a minor improvement since "private enterprise numbers aren't yet at 16 bits" and collisions were not that possible. Later on freeradius-client completely removed `VENDOR()` as part of a different way of handling vendor ids.
Also radcli uses 16 bits for shifting vendor id as seen here https://github.com/radcli/radcli/blob/f0d1a1704595d62182d6a03c980ed07e906374...
so it made more sense to me, to keep 16 bits shifting to both libraries.
I should note, that adding `VENDOR()` is more of a quick workaround at this point to allow kamailio to compile against both libraries with minimum effort. Otherwise a different implementation should be used by kamailio if freeradius-client is used