As I previously described on the forum, Zebra is sending RAs and thus causes conflicts with the RAs sent by radvd, as they don't adhere to the config of Vyos.
This basically breaks any setup that relies on router advertisements to send out DNS servers, as the systems receiving the invalid RAs tend to misbehave in various weird ways, depending on the OS. (Ranging from taking forever to accept the DNS servers to just ignoring them entirely)

A somewhat simple workaround that seems to (mostly) fix issues caused by this is to increase the RA priority of the radvd RAs for all relevant interfaces, by setting the default-preference to high. But this removes the ability to use the priority for any other purpose.