The way I do it is by using the user level systemd service.
I think latest vdagent improved this by making their user level service
depend on graphical user session.
See:
The one in Debian now is still outdated:
/usr/lib/systemd/user/spice-vdagent.service
So you can probably patch that manually with the above and see if that helps.
Regards,
Shmerl.