I see your point, but generally, you want to use uRPF by receiving a BGP feed with prefixes to block, with a specific next hop, so you need to statically route this nexthop to blackhole, so all the prefixes are blackholed and uRPF can kick in.

Does that mean uRPF is useless in VyOS, or is there a workaround I failed to find ?

The version and the relevant lines of configuration are at the beginning.
Am I missing something ?

I am sorry, this is bug, I hardly see why it should be a support question.