You cannot just put an arbitrarily-large attenuation after a VNA port and expect reasonable results. Roger Need's suggestion of 20dB is about as high as I would go; keep in mind that the round-trip loss is double the attenuation so the signal quickly becomes uselessly noisy.
And, yes, you can use a noise bridge, BITD there were some high-end ones made commercially.
Best regards.
Donald S Brant Jr