Quote from https://www.qubes-os.org/doc/qrexec3/ :
"""
In Qubes 4.0 policy confirmation dialog (ask action) allow the user to specify target VM.
User can choose from VMs that, according to policy, would lead to ask or allow actions. It is not possible to select VM that policy would deny.
By default no VM is selected, even if the caller provided some,
but policy can specify default value using default_target= parameter.
"""
So, you need something like:
$anyvm ssh-vault ask,default_target=ssh-vault
Quote from https://www.qubes-os.org/doc/qrexec3/ :
"""
In Qubes 4.0 policy confirmation dialog (ask action) allow the user to specify target VM.
User can choose from VMs that, according to policy, would lead to ask or allow actions. It is not possible to select VM that policy would deny.
By default no VM is selected, even if the caller provided some,
but policy can specify default value using default_target= parameter.
"""
So, you need something like: